Purpose: Punctuation is traditionally seen to represent grammatical structures in writing, but some authors argue that it can also reflect the intonation and pauses of speech. In two experiments, we examined the influence of grammar and prosody on adults' judgments of comma placement. Method: University students rated the appropriateness of commas in various sentence structures. These included sentences in which the subject and verb were disrupted by a comma. These commas were ungrammatical, but prosodically appropriate, in that they marked where a slight pause could naturally occur in speech. Results: In Experiment 1, 192 students rated ungrammatical commas as more appropriate if they occurred at prosodic pause points than at no-pause points. In Experiment 2, 235 students rated prosodically appropriate commas as more appropriate if they were also grammatical than if they were not. Overall, participants with better written language skills distinguished more between grammatical and ungrammatical commas, but acceptance of prosodically appropriate commas occurred across levels of written language skill. Conclusion: Literate adults use prosody, as well as grammar, to judge the appropriateness of comma use. These findings have implications for theories of punctuation, and can inform debate on students' and teachers' treatment of punctuation.