The considerable growth in the number of smart mobile devices with a fast Internet connection provides new challenges for survey researchers. In this article, I compare the data quality between two survey modes: self-administered web surveys conducted via personal computer and those conducted via mobile phones. Data quality is compared based on five indicators: (a) completion rates, (b) response order effects, (c) social desirability, (d) non-substantive responses, and (e) length of open answers. I hypothesized that mobile web surveys would result in lower completion rates, stronger response order effects, and less elaborate answers to open-ended questions. No difference was expected in the level of reporting in sensitive items and in the rate of non-substantive responses. To test the assumptions, an experiment with two survey modes was conducted using a volunteer online access panel in Russia. As expected, mobile web was associated with a lower completion rate, shorter length of open answers, and similar level of socially undesirable and non-substantive responses. However, no stronger primacy effects in mobile web survey mode were found.