Examination of the mediating mechanisms within the self-determination theory process model will provide deeper insight in the mechanisms of motivational regulations and psychological need satisfaction. Optimally, such studies should also include examination of action theory links and conceptual theory links to consider theory capacity (Cerin & MacKinnon, 2009). We studied 318 participants (aged 23-67 years) included in a controlled trial testing a digital intervention tool aiming to promote self-determined exercise motivation using 3 wave measurement over the course of 6 weeks. The participants (279 women and 40 men) were randomized into experimental and control group and completed a web-based test battery with the SDT-related measures (Psychological needs in exercise scale, Behavioral regulations in exercise scale -2, and Leisure time exercise questionnaire) at baseline, post intervention (3 weeks) and follow up (6 weeks). Mediation analyses were conducted using the SPSS macro Process by Hayes (2013). Results showed indirect effects of psychological need satisfaction regarding the effect of the intervention on motivation and exercise behavior at follow up measures. In the group of participants exposed to the intervention, higher levels of autonomy need satisfaction at follow-up predicted lower levels of amotivation and external regulation. In these models we found positive significant action theory links and negative conceptual theory links, showing the intervention to impact autonomy in positive direction, while higher autonomy need satisfaction in turn decreased controlled motivation and amotivation. Higher scores of autonomy need satisfaction at follow-up also predicted higher scores of identified regulation, intrinsic regulation and autonomous motivation. Lower levels of amotivation were linked with higher levels of total exercise. In sum, the results reflect expectations from a a self-determination theory perspective and are in favor of intervention efficacy.