Evaluating knowledge and learning in psychotherapy is a growing field of research. Studies that develop and evaluate valid tests are lacking, however. Here, in the context of internet-based cognitive behavioral therapy (ICBT) for adolescents, a new test was developed using subject matter experts, consensus among researchers, self-reports by youths, and a literature review. An explorative factor analysis was performed on 93 adolescents between 15 and 19 years old, resulting in a three-factor solution with 20 items, accounting for 41% of the total variance. The factors were "Act in aversive states," "Using positive reinforcement," and "Shifting attention." The internal consistency for the whole instrument was good ([alpha] = 0.84). Hopefully, the procedure presented here can contribute to the field by illustrating one way of evaluating knowledge in ICBT. The clinical use of the knowledge test needs further investigation.