['A strong clinical rationale exists to hypothesize that neuropsychological impairment interferes with treatment-initiated change, thereby leading to poor outcome. This study examined the relationship of executive function impairment, change process factors and substance use outcomes in a sample of substance users in intensive 12-step treatment.', 'Participants were 118 individuals entering residential or intensive day treatment at two traditional treatment programs. Participants were assessed at entry into treatment, at discharge from treatment, and at 1 and 6 months following treatment. Participants were administered a battery of measures to assess executive function impairment, processes hypothesized to mediate change in 12-step treatment, negative prognostic indicators and substance use outcomes.', 'More than half the sample showed some form of executive function impairment. Executive function impairment did not directly predict worse substance use outcomes nor difficulty acquiring or maintaining change processes. However, impairment significantly moderated the relationship between change processes and outcome. Change processes were strongly related to outcome for unimpaired individuals but weakly related for impaired individuals.'] Do effects of executive function impairment on change processes and substance use outcomes in 12-step treatment?