ACS Citation Format: A Complete Guide
The American Chemical Society (ACS) citation style is the standard for chemistry, biochemistry, and many related physical sciences. It uses numbered in-text markers and a sequentially ordered reference list — a system built for precision and readability in technical literature.
What is ACS citation style?
ACS style is published by the American Chemical Society and documented in The ACS Style Guide: Effective Communication of Scientific Information (3rd edition). It is used in ACS journals such as the Journal of the American Chemical Society, Analytical Chemistry, and ACS Nano, and is also adopted across many university chemistry departments and other science publishers.
The style is designed for technical scientific writing where brevity and unambiguous source identification matter. Its defining feature is the numbered citation system: sources receive a number when first cited, and that same number is reused every time the source appears again. This keeps the body text clean and lets readers jump directly to the numbered entry in the reference list.
ACS style also permits an author-date system (similar to APA), but numbered references are by far the most common format used in ACS publications and are the focus of this guide.
In-text citations
ACS in-text citations are numbers, not author names or dates. There are two accepted formats:
- Superscript numbers — the most widely used format in ACS journals. The number appears as a raised character directly after the punctuation or at the end of the relevant clause.
- Italic numbers in parentheses — an alternative accepted in some ACS journals and many university submissions.
Numbers are assigned in the order that sources first appear in the text. If the same source is cited again later, it keeps its original number. If you cite multiple sources in one place, list their numbers together separated by commas or an en dash for a range.
Transition-metal catalysis has enabled a wide range of C–H functionalization reactions.1,2 Chen and colleagues later extended this approach to asymmetric systems.3
Transition-metal catalysis has enabled a wide range of C–H functionalization reactions (1, 2). Chen and colleagues later extended this approach to asymmetric systems (3).
Citing the same source more than once
Once a source has been assigned a number, that number is used every subsequent time the source is cited. Do not assign a new number to a source you have already cited.
Citing a specific page or location
ACS style does not commonly require page numbers in in-text citations the way APA or MLA do. When you need to direct a reader to a specific page — for instance, when quoting directly — you may add the page after the number: 4 (p 312) or (4, p 312).
Reference list rules
The reference list appears at the end of the paper under the heading References. Every entry is numbered to match the in-text citation, and entries are listed in the order they were first cited — not alphabetically.
Author names
Author names follow the format: Last Name, Initials. First and middle names are reduced to initials with no spaces between them. Multiple authors are separated by semicolons. When a work has more than a certain number of authors, some journals allow truncation with "et al.", but the ACS Style Guide recommends listing all authors.
One author: Smith, J. A.
Two authors: Smith, J. A.; Jones, B. C.
Three authors: Smith, J. A.; Jones, B. C.; Williams, D. E.
Journal name abbreviations
Journal titles are always abbreviated in ACS references, following the CASSI (Chemical Abstracts Service Source Index) standard abbreviations. The abbreviated title is set in italic. For example:
- Journal of the American Chemical Society becomes J. Am. Chem. Soc.
- Analytical Chemistry becomes Anal. Chem.
- Nature Chemistry becomes Nat. Chem.
- ACS Nano stays ACS Nano (no abbreviation needed)
DOIs
When a DOI (Digital Object Identifier) is available, include it at the end
of the reference as a full URL in the form
https://doi.org/10.xxxx/yyyyy. The ACS Style Guide recommends
including DOIs for all journal articles where they exist.
Journal articles
Journal articles are the most common source type in chemistry writing. The general ACS format is:
Author1, A. A.; Author2, B. B.; Author3, C. C. Abbreviated Journal Name Year, Volume (Issue), first page–last page. DOI.
Note that the year is in bold, the volume number is in italic, and the issue number appears in regular weight inside parentheses. Article titles are not included in the traditional ACS journal reference format.
(1) Hartwig, J. F.; Larsen, M. A. Undirected, Homogeneous C–H Bond Functionalization: Challenges and Opportunities. ACS Cent. Sci. 2016, 2 (5), 281–292. https://doi.org/10.1021/acscentsci.6b00032
(2) Chen, M. S.; Prabagaran, N.; Labenz, N. A.; White, M. C. Serial Ligand Catalysis: A Highly Selective Allylic C–H Oxidation. J. Am. Chem. Soc. 2005, 127 (19), 6970–6971. https://doi.org/10.1021/ja0506573
Online-only journals without page numbers
For articles published online ahead of print or in journals that use article numbers rather than page ranges, include the article number in place of pages:
(3) Li, X.; Wang, H.; Zhang, Y. ACS Nano 2023, 17 (8), e12345. https://doi.org/10.1021/acsnano.3c00000
Books and book chapters
Entire book
For a book, include the author(s) or editor(s), the title in italic, the edition if not the first, the publisher, the city of publication, and the year.
Author, A. A. Book Title, Nth ed.; Publisher: City, Year; pp xxx–xxx.
(4) Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012.
Chapter in an edited book
When citing a single chapter, name the chapter author(s) first, then introduce the book with "In," followed by the book title, the editor(s), and the publication details. Include the page range of the chapter.
Chapter Author, A. A. In Book Title; Editor, B. B., Ed.; Publisher: City, Year; pp xxx–xxx.
(5) Togni, A. Enantioselective Catalysis with Metal Complexes. In Transition Metals for Organic Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, 2004; pp 15–55.
Online sources and websites
Web pages, databases, and online reports that are not formally published journals or books follow a simpler format. Because web content can change or disappear, an access date is required.
Author, A. A. Title of Page or Document. URL (accessed Month Day, Year).
(6) National Institute of Standards and Technology. NIST Chemistry WebBook. https://webbook.nist.gov/ (accessed March 10, 2026).
(7) U.S. Environmental Protection Agency. Integrated Science Assessment for Particulate Matter. https://www.epa.gov/isa (accessed January 5, 2026).
Theses and dissertations
Graduate theses follow a specific format that identifies the degree level, the institution, and the year. The thesis title appears in italic.
Author, A. A. Title of Thesis. Degree Type Thesis, University Name, City, Year.
(8) Johnson, R. K. Development of Novel Palladium Catalysts for Cross-Coupling Reactions. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 2021.
(9) Park, S. Y. Synthesis and Characterization of Functionalized Carbon Nanotubes. M.S. Thesis, University of California, Berkeley, CA, 2019.
Common mistakes to avoid
Listing references alphabetically instead of by order of citation
Unlike APA or MLA, ACS references are not alphabetized. They must appear in the order their corresponding numbers appear in the text. If source 3 in your list was actually cited after sources 1 and 2, it belongs at position 3 — regardless of the author's last name.
Reusing a number for a different source
Each source gets one number, and that number belongs to that source alone. If you cite a new source for the first time, it must receive the next available number — never a number already assigned to a different reference.
Writing out journal names in full
ACS style requires abbreviated journal names in the reference list. Writing "Journal of the American Chemical Society" instead of "J. Am. Chem. Soc." is a formatting error. Use the CASSI database to find the correct abbreviation for any journal.
Omitting the DOI
For any journal article that has a DOI, the DOI URL should appear at the end of the reference. Leaving it out makes the citation harder to verify and is not consistent with current ACS guidelines.
Incorrect author initial format
Author names in ACS use initials without spaces: "Smith, J. A." — not "Smith, J. A. B." with spaces, and not "Smith, John A." with the full first name spelled out. Each initial is followed by a period.
Missing access date for web sources
Online sources that are not journal articles require an access date in parentheses at the end of the entry. Without it, the citation cannot establish when the version was viewed, which matters if the content changes.
Quick summary
| Feature | ACS Rule |
|---|---|
| In-text format | Superscript numbers or italic numbers in parentheses |
| Numbering | Sequential by order of first appearance in the text |
| Reference list order | Numbered order — not alphabetical |
| Author format | Last Name, Initials. (e.g., Smith, J. A.) |
| Multiple authors | Separated by semicolons |
| Journal names | Abbreviated and italic (CASSI standard) |
| Year | Bold |
| Volume | Italic |
| DOI | Included as https://doi.org/... when available |
| Web access date | Required for non-journal online sources |
ACS style is precise and consistent. Once you understand its core logic — numbers assigned in citation order, abbreviated journal names, and authors by initials — the rest of the rules fall into place. When in doubt, consult the journal's own author guidelines, as individual ACS publications may have minor variations.
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