This paper reports on an experimental test of coercion theory early onset model of delinquency. Results are from the Oregon Divorce Study-II, a randomized preventive intervention trial with a sample of 238 recently separated mothers and their sons in early elementary school. The objective was to experimentally manipulate parenting variables hypothesized to influence development of delinquent behaviors. Multiple-method assessment spanned 36 months. Because the intervention focused on parent training, we expected that any intervention effects on changes in child outcomes would be mediated by hypothesized intervening mechanisms. Linear growth models showed significantly greater reduction in boys' delinquency and deviant peer affiliation in the experimental group relative to the controls. Subsequent models using no method overlap in constructs demonstrated that the intervention effect on delinquency operated through growth in parenting and reduction in deviant peer affiliation.