Siblings' constructive and unstructured shared activities were examined as moderators of the links between first- and second-born siblings' adjustment across a two-year period in adolescence. Siblings (N = 189 dyads) reported on their depression, peer competency, self worth during home interviews, and their time together in constructive (e.g., sports, hobbies) and unstructured (e.g., hanging out) activities during seven nightly phone interviews. Siblings spent an average of 10 hours together across seven days, about 12% in constructive and 25% in unstructured activities. Regression analyses revealed that, controlling for adjustment at Time 1, associations between siblings' adjustment scores were moderated by siblings' constructive and unstructured shared time. These patterns were most evident in mixed-sex dyads. (Contains 2 tables and 4 figures.)