Abstract: Objective: To explore the explanatory power of a self-determination theory (SDT) model of health behaviour change for hearing aid adoption decisions and fitting outcomes. Design: A quantitative approach was taken for this longitudinal cohort study. Participants completed questionnaires adapted from SDT that measured autonomous motivation, autonomy support, and perceived competence for hearing aids. Hearing aid fitting outcomes were obtained with the international outcomes inventory for hearing aids (IOI-HA). Sociodemographic and audiometric information was collected. Study sample: Participants were 216 adult first-time hearing help-seekers (125 hearing aid adopters, 91 non-adopters). Results: Regression models assessed the impact of autonomous motivation and autonomy support on hearing aid adoption and hearing aid fitting outcomes. Sociodemographic and audiometric factors were also taken into account. Autonomous motivation, but not autonomy support, was associated with increased hearing aid adoption. Autonomy support was associated with increased perceived competence for hearing aids, reduced activity limitation and increased hearing aid satisfaction. Autonomous motivation was positively associated with hearing aid satisfaction. Conclusion: The SDT model is potentially useful in understanding how hearing aid adoption decisions are made, and how hearing health behaviour is internalized and maintained over time. Autonomy supportive practitioners may improve outcomes by helping hearing aid adopters maintain internalized change.