Homeless adolescents who used alcohol or illicit substances but were not seeking treatment (n = 54) were recorded during brief motivational interventions. Adolescent language during sessions was coded on the basis of motivational interviewing concepts (global ratings of engagement and affect, counts of commitment to change, statements about reasons for change, and statements about desire or ability to change), and ratings were tested as predictors of rates of substance use over time. Results indicate that statements about desire or ability against change, although infrequent (M = 0.61 per 5 min), were strongly and negatively predictive of changes in substance use rates (days of abstinence over the prior month) at both 1- and 3-month postbaseline assessment (ps < .001). Statements about reasons for change were associated with greater reductions in days of substance use at 1-month assessment (p < .05). Commitment language was not associated with outcomes. Results suggest that specific aspects of adolescent speech in brief interventions may be important in the prediction of change in substance use. These relationships should be examined within larger samples and other clinical contexts.