The main objectives of this study are to evaluate the efficacy and mechanisms of change of an Internet-delivered Rational Emotive Behavior Therapy for adolescents (12-15 years) diagnosed with a principal anxiety or depressive disorder according to DSM-5 criteria. Participants will be randomly allocated to one of the two groups: experimental (REBTonAd) and waitlist control. The intervention is structured in 8 modules and will be delivered over six weeks. Assessments will be conducted at four time points: baseline, post-treatment, 3-month follow-up, and 6-month follow-up.
