The present study was designed to disentangle and define the unique contributions of family form and family processes to substance use and association with deviant peers among adolescents. Eighty adolescents and their mothers participated in the study; half were from divorced single-parent families and half were from families in which the parents were still married. Maternal assessments of interparental conflict and the quality of the mother-adolescent relationship were used to prospectively forecast adolescent reports of substance use and affiliation with deviant peers one year later. The results revealed that mother-adolescent conflict predicted adolescent alcohol use, and maternal acceptance-rejection predicted both drug use and affiliation with deviant peers. These findings support the hypothesis that family processes rather than family form forecast adolescents' substance use and deviant peer affiliation. Clinical implications of the findings are discussed.