Due to a rising mobile device penetration, Web surveys are increasingly accessed and completed on smartphones or tablets instead of desktop computers or laptops. Mobile Web surveys are also gaining popularity as an alternative self-administered data collection mode among survey researchers. We conducted a methodological experiment among iPhone owners and compared the participation and response behavior of three groups of respondents: iPhone owners who started and completed our survey on a desktop or laptop PC, iPhone owners who self-selected to complete the survey on an iPhone, and iPhone owners who started on a PC but were requested to switch to iPhone. We found that respondents who completed the survey on a PC were more likely to be male, to have a lower educational level, and to have more experience with Web surveys than mobile Web respondents, regardless of whether they used the iPhone voluntarily or were asked to switch from a PC to an iPhone. Overall, iPhone respondents had more missing data and took longer to complete the survey than respondents who answered the questions on a PC, but they also showed less straightlining behavior. There are only minimal device differences on survey answers obtained from PCs and iPhones.