In web surveys, no interviewer is present to clarify question comprehension problems, which can be particularly prevalent among respondents with low literacy skills. Although plain language is used in various contexts to improve text comprehensibility, its use in social science questionnaires has not been investigated to date. Using a between-subjects design, we compared the use of plain language and standard language in a web questionnaire in terms of data quality and respondents' evaluation of the questionnaire. Plain language did not generally have a positive effect on data quality and questionnaire evaluation. However, in the plain-language condition, we found less item nonresponse and more response differentiation among respondents who spoke a language other than German at home, which suggests that especially respondents who likely have low literacy skills benefit from plain language. Based on our findings, we outline opportunities for future research and give practical recommendations.