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An in-text citation in American Chemical Society (ACS) style can take one of three forms: a superscript number, an italic number in parentheses, or the author’s name with the year. You choose one method for the whole document and use it throughout. The number-based forms link to a reference list ordered by first citation; the author-date form links to a list ordered alphabetically by last name. This guide sets the three side by side, with invented examples, so you can pick the one that suits a manuscript or thesis.
The format: three methods and how each works
University library guides that follow ACS conventions describe the same three options: superscript numbers, italic numbers in parentheses, and author name plus year. All of them say to select one and apply it consistently. The table below combines what the Towson University, Ohio University and University of South Carolina library guides state (see Sources).
| Method | Marker in the text | Reference list order | Several references at once |
|---|---|---|---|
| Superscript number | A raised number placed after the cited words, such as a sentence ending 9 | Numbered in order of first citation | Separate numbers with a comma; for three or more in sequence, give the first and last joined by a dash |
| Number in parentheses (italic) | The number in parentheses, in italics, such as (9) | Numbered in order of first citation | Separate numbers with a comma |
| Author and year | The author’s surname and the year, such as (Axelrod, 2003) | Alphabetical by the first author’s last name | List the references alphabetically inside one set of parentheses, separated by semicolons |
Reusing a reference
In the numbered methods, a reference keeps the same number every time you cite it. You do not issue a new number on the second or third use. UBC Library’s ACS quick guide says the superscript number is repeated each time the same reference is used.
Author names in the author-date method
One author: the surname and year. Two authors: join the surnames with “and”. Three or more: the first author’s surname followed by “et al.” and the year.
Examples
The names, findings and years below are invented to show the patterns. They are not real sources.
- Superscript: Okafor et al. observed a similar shift in yield under the same conditions.4
- Italic number in parentheses: Rivera and Lindqvist reported related findings (4).
- Author and year: The same shift has been reported previously (Okafor et al., 2019).
- Two authors, author-date: (Rivera and Lindqvist, 2021).
- Multiple references, author-date: (Okafor et al., 2019; Rivera and Lindqvist, 2021).
In-text citation version for a run of references
When one statement rests on several sources, the numbered methods use a comma between non-consecutive numbers, such as 1,2 or (1, 2). Where three or more are consecutive, give the first and last with a dash, such as 1-3. In author-date style, place the sources in alphabetical order within one pair of parentheses and separate them with semicolons, as in the last example above. Example reference numbers here are illustrative only.
Common errors
- Mixing methods. Using superscripts in one chapter and author-date in the next breaks the rule to select one method and apply it throughout.
- Renumbering a reference on reuse. A source cited again keeps its original number.
- Wrong list order for the chosen method. Numbered methods run in order of first citation; author-date lists run alphabetically. Switching the in-text form without reordering the list leaves the two out of step.
- “et al.” for two authors. Two authors are joined with “and”; “et al.” begins at three.
- Listing author-date sources out of order. Several sources in one set of parentheses are alphabetical and separated by semicolons.
For manuscripts and theses
For a journal submission, the target journal sets the form it wants, so read its author instructions before you format. For a thesis, confirm the form with your institution or supervisor, then apply it from the first chapter to the last. Our guide to chemistry manuscript writing conventions in ACS style covers the wider manuscript picture, and citation styles and referencing formats compares ACS with the other major styles. ACS is also not the same as CSE; see our guide to science reference style for that system, and how to cite a lab manual for a common source type in teaching labs.
Style-guide edition
UBC Library’s ACS quick guide, last updated 25 August 2026, states that it is based on the updated 2026 edition of the ACS Guide to Scholarly Communication. It adds that superscripts remain a preferred method, with parenthetical and author-date referencing as other options. The three-method pattern described on this page is the one the Towson, Ohio University and University of South Carolina guides give; those guides were last updated between October 2025 and August 2026.
FAQ
How do you do an in-text citation in ACS style?
Pick one of three methods and use it throughout: a superscript number, an italic number in parentheses, or the author’s surname and year. Numbers point to a reference list in order of first citation; author-date points to an alphabetical list.
What is the American Chemical Society in-text citation format?
It is the same set of three options. The American Chemical Society’s convention is to cite each source in the text by number or by author and year, linked to the reference list. UBC Library describes superscripts as a preferred method.
Does ACS use numbers or author-date?
Both are accepted. Two number forms and one author-date form are available, and the choice is yours unless a journal or institution specifies one.
How do you cite the same source twice?
In the numbered methods, reuse the same number.
How do you cite several sources at once?
Separate numbers with commas, use a dash for three or more in sequence, or in author-date style list the sources alphabetically in one set of parentheses separated by semicolons.
Sources
Each source below was checked on 29 October 2026.
- Towson University, Albert S. Cook Library, ACS Style guide: https://towson.libguides.com/acsstyle (page last updated 4 March 2026).
- Ohio University Libraries, Citation guide: ACS: https://libguides.library.ohio.edu/citation/acs (page last updated 28 October 2025).
- University of South Carolina Libraries, Science citation styles: ACS: https://guides.library.sc.edu/sciencecitationstyles/acs (page last updated 14 August 2026).
- UBC Library, ACS quick guide: https://guides.library.ubc.ca/acsquickguide (page last updated 25 August 2026; edition statement).
