The recruitment of participants for online survey research presents many challenges. In this work, we present four experiments examining how two different kinds of “surprise” financial incentives affect the rate of participation in a longitudinal study when participants are initially solicited with either an appeal to intrinsic motivation to participate in research or one that also offers extrinsic financial incentives. We find that unexpected financial incentives (“existence surprises”) presented to people who click a recruitment advertisement focused on intrinsic incentives lead to a lower recruitment rate than do the same incentives offered to those who clicked an advertisement that led them to expect it. However, when potential participants expect a financial incentive, surprising them with a higher amount (“amount surprises”) yields a higher recruitment rate. We interpret these results in the context of crowding theory. Neither type of surprise affected ongoing participation, measured as the number of questions and questionnaires completed over the course of the study.