Previous studies have not found effective ways of encouraging participants to use smartphones to complete web surveys. We hypothesize that conditional differential incentives (the amount depending on the device the respondent uses to complete the web survey) can increase overall participation rates and the proportion of respondents who use a particular device in web surveys. We conducted an experiment using a volunteer online access panel in Russia with 5,474 invitations sent to regular mobile Internet users. We varied the invitation mode (SMS vs. e-mail) and encouragement to use a particular device for completing the survey – mobile phone or personal computer (PC). SMS increased the proportion of mobile web respondents, while e-mail increased the proportion of PC web respondents. As expected, differential incentives increased the overall participation rates by 8-10 percentage points if higher incentives were offered for completing the survey on a mobile phone. Contrary to expectations, offering higher incentives to PC web respondents did not produce higher participation rates compared to the control condition. Both encouraging the use of a mobile phone and offering higher incentives were effective at increasing the proportion of respondents using mobile devices. In terms of both participation rates and the proportion of respondents using mobile devices, offering incentives 50% higher was as efficient as offering incentives 100% higher for mobile web respondents. Offering higher incentives to mobile web respondents also had an effect on sample composition. Significantly higher participation rates were found among females and those with higher education.