High dropout rates are still a problem with online training. It is strongly suggested that learner characteristics influence the decision to persist in an online course or to drop out. The study explored the differences in domain-specific prior knowledge, motivation, computer attitude, computer anxiety, and learning skills between dropouts and active learners who enrolled in a vocational online training about media pedagogy for teachers. The data were collected from 575 trainee teachers from which three groups were formed: (a) students who only registered (n = 72) and (b) students who started learning but failed to complete any of the course modules (n = 124) and (c) active students who completed at least one module (n = 379). A dropout rate of 34.1% was observed. In general, only small effects were found. Students dropping out were older, had less prior knowledge, and lower skills in arranging an adequate learning environment.