In recent years, an increasing number of online panel participants respond to surveys on smartphones. As a result, survey practitioners are faced with a difficult decision: Either they hold the questionnaire design constant over time and thus stay with the original desktop-optimized design; or they switch to a smartphone-optimized format and thus accommodate respondents who prefer participating on their smartphone. Even though this decision is all but trivial, little research thus far has been conducted on the effect of such an adjustment on panel members’ survey participation and device use. We report on the switch to a smartphone-optimized design in the German Internet Panel (GIP), an ongoing probability-based online panel that started in 2012 with a desktop-optimized design. We investigate whether the introduction of a smartphone-optimized design affected overall response rates and smartphone use in the GIP. Moreover, we examine the effect of different ways of announcing the introduction of the smartphone-optimized design in the invitation email on survey participation using a smartphone.