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The Physical Activity Counseling (PAC) randomized control trial examined the effects of a physical activity counseling intervention on physical activity motivation and on physical activity behavior. Results have shown that both quantity and quality motivation influenced physical activity levels at the end of the PAC intervention, however their combined influence has not been investigated and theory and past research point to a possible moderated mediation model. Thus the current study examined if self-determined motivation moderated the mediational influence of quantity of motivation on the relationship between the intervention and physical activity in the PAC trial.Physically inactive primary care patients (n = 120) were recruited and randomized to an intensive counseling or brief counseling group. All patients responded to a quantity of motivation scale (% of motivation for physical activity from 1 to 7 days) and a self-determined motivation scale (BREQ-2) at 6 weeks (mid intervention) and the Godin Leisure-Time Exercise Questionnaire at 13 weeks (end intervention). Moderated mediation was tested using multiple regression analysis.Regression analyses showed that all criteria for mediation were met and specifically that the intervention–physical activity relationship became non-significant with the inclusion of quantity of motivation (β = .22, p < .05 to β = .13, p > .05). Furthermore, the relationship between quantity of motivation and physical activity was moderated by self-determined motivation (β = .17, p < .05). The moderation occurred at an RAI score of 22 and higher (p < .05).Findings confirm the importance of distinguishing quantity and quality of motivation and support Self-Determination Theory by showing significant moderation by self-determined physical activity motivation. Results demonstrate the importance of moderated mediation models to determine complex interactions between psychological constructs and physical activity. This knowledge can help increase the efficacy of physical activity interventions.