Offering participants in mobile app studies personalized feedback on the data they report seems an obvious thing to do: Participants might expect an app to provide feedback given their experiences with commercial apps, feedback might motivate more people to participate in the study, and participants might be more motivated to provide accurate data so that the feedback is more useful to them. However, personalized feedback might lead participants to change the behavior that is being measured with the app, is costly to implement, and also constrains other design decisions for the data collection. In this article, we report on an experimental study that tested the effects of providing personalized feedback in a 1-month mobile app–based spending study. Based on the app paradata and responses to a debrief survey, it seems that participants reacted positively to the feedback. The feedback did not have the potential negative effect of altering the spending participants reported in the app. However, the feedback also did not have the intended effect of increasing initial participation or ongoing adherence to the study protocol.