Web surveys are being increasingly incorporated into national survey data collection programs in the United States because of their cost/time-efficiencies. Yet, response rates and data quality issues in web surveys remain important challenges. As a basic study designed to better understand data quality in a mixed mode national survey, this article investigates the degree to which web versus mail survey modes affect unit and item responses. Findings indicate that the web survey mode produces a lower unit response rate compared to the mail mode. However, the web mode elicits higher data quality in terms of item responses to both closed- and open-ended questions. These mode effects on data quality remain after sociodemographic variables are held constant. Given the increasing integration of web survey questionnaires into mixed mode studies, additional research is necessary to understand and document the processes that underlie mode differences when responding to self-administered surveys.