This paper offers a critical look at the promises and drawbacks of a popular, novel data collection technique—online tracking—from the point of view of sample composition. Using data from two large-scale studies about political attitudes and information consumption behavior carried out in Germany and Switzerland, we find that the likelihood of participation in a tracking study at several critical dropout points is systematically related to the gender, age, and education of participants, with men, young, and more educated participants being less likely to dropout of the studies. Our findings also show that these patterns are incremental, as changes in sample composition accumulate over successive study stages. Political interest and ideology were also significantly related to the likelihood of participation in tracking research. The study explores some of the most common concerns associated with tracking research leading to non-participation, finding that they also differ across demographic groups. The implications of these findings are discussed.