The time-varying influences of peer and family support on adolescent daily mood were explored among youth transitioning from middle school to high school (8th to 9th grade, N = 268) as compared to youth transitioning from 10th to 11th grade (N = 240). Real-time measures of daily positive and negative affect (ecological momentary assessments) were collected via palmtop computers at baseline, 6 months, and 12 months. Participants rated 12 mood adjectives in response to 5 to 7 random prompts per day for 7 consecutive days. Perceived peer and family support were assessed via self-report. Mixed-effects regression analyses revealed significant grade by time by peer support interactions for positive and negative mood, with the younger cohort showing greater increases in the relation between peer support and affect over time than the older cohort. Family support did not interact with cohort or time.