This article investigates unintended mobile access to surveys in online, probability-based panels. We find that spontaneous tablet usage is drastically increasing in web surveys, while smartphone usage remains low. Further, we analyze the bias of respondent profiles using smartphones and tablets compared to those using computers, on the basis of several sociodemographic characteristics. Our results indicate not only that mobile web respondents differ from PC users but also that tablet users differ from smartphone users. While tablets are used for survey completion by working (young) adults, smartphones are used merely by the young. In addition, our results indicate that mobile web respondents are more progressive and describe themselves more often as pioneers or forerunners in adopting new technology, compared to PC respondents. We further discover that respondents’ preferences for devices to complete surveys are clearly in line with unintended mobile response. Finally, we present a similar analysis on intended mobile response in an experiment where smartphone users were requested to complete a mobile survey. Based on these findings, testing on tablets is strongly recommended in online surveys. If the goal is to reach young respondents, enabling surveys via smartphones should be considered.