Relevance & Research Question: Research on cross-sectional surveys has shown that prepaid, unconditional incentives are more effective than postpaid incentives. However, no such evidence is available for self-administered longitudinal mixed-mode surveys of the general population. With the emergence of probability-based panels in the social sciences, identifying the optimal incentive timing strategy (prepaid, postpaid) to increase panel survey participation becomes paramount. In our presentation, we will address the research question whether prepaid monetary incentive is superior to postpaid monetary incentive in terms of panel survey participation. Methods & Data: We use data from the recruitment phase of the GESIS Panel – a probability-based mixed-mode panel (online, offline using mailed surveys) of the German population. Respondents had been recruited offine (CAPI surveys) and are invited to self-administered surveys every second month. Recruitment took place during May and December 2013. We conducted a quasi-experiment encompassing two groups. The control group (n=4340) consisted of respondents who answered the first regular self-administered survey in 2013. They were promised an incentive of 5 Euro for survey participation. To redeem the incentive, they had to provide their bank account data. The experimental group (n=589) consisted of respondents who answered their first self-administered survey in January 2014. They received a five Euro bill enclosed into a mailed invitation. Results: First analyses show a substantial increase of survey participation from 77% in the control group to 90% in the experimental group (z = -7.21, p< 0.01). A separate analysis of online and offline respondents show that the increase is more pronounced in the group of offline respondents (66% control, 88% experimental) in comparison to online respondents (84% control, 93% experimental). Besides participation rates, the use of prepaid incentives send out by letter had a positive side effect for panel maintenance, because respondents were highly motivated to keep their address information up to date. Added Value: The findings suggest that prepaid incentives increase panel survey participation rates compared to promised incentives in a probability-based mixed-mode context. Moreover, sending out prepaid incentives tend to increase panelists´ motivation to keep their contact addresses up to date, and hence, facilitating panel maintenance.