In recent years emotion socialization theory (Eisenberg, Cumberland, & Spinrad, 1998) has begun to be used in parenting interventions, allowing an important and effective method for testing the theory. The current study is one such example, and examined moderators of program effects and mechanisms of change in an emotion-focused group parenting program, Tuning in to Teens (TINT), to determine whether an intervention with this theoretical approach would be effective in improving adolescent internalizing difficulties. Schools were randomized into intervention and control conditions. Data was collected from 225 parents and 224 youth during the young person's final year of elementary school (6th grade) and again, 10 months later in their first year of secondary school (7th grade). Those in the intervention condition received a 6-session program targeting parent emotion awareness/regulation, parental beliefs about emotion and parents' emotion coaching skills. Multilevel analyses were conducted to examine moderators of the intervention and regression analyses were conducted to examine mediators of program effects. Results showed greater benefits for intervention subgroups with high preintervention scores on youth anxiety. Parental internalizing difficulties and parental difficulties in emotion awareness/regulation did not moderate program effects. Mediation analyses supported emotion socialization theory and showed parents' who participated in the TINT parenting program reported improvements in their own awareness/regulation and emotion socialization, which were, in turn, related to reductions in youth internalizing difficulties.