Background: Adolescents' physical activity (PA) choices may involve reasoned thought, but also may be impulsive, with little reflection. Furthermore, the outcomes adolescents expect from PA (e.g. look good, have fun) can influence their impetus to engage in, or avoid, active pursuits. The purpose of this study was to explore older adolescents' reflective and impulsive thoughts about health- and social/appearance-related PA outcomes and investigate how those thoughts relate to their PA behaviour. Method: One hundred and thirty-seven undergraduate students (103 female; 34 male) aged 17-19 (M=18.12, SD=0.65) completed a Go/No-Go Association Task (GNAT; Nosek & Banaji, 2001) that assessed automaticity (via response times) of associations between PA words and either health outcomes (e.g. energetic, happy, strong) or social/appearance outcomes (e.g. thin, popular, muscular). Participants also completed questionnaires assessing PA behaviour, attitudes, outcome expectations, and physical self-perceptions. Results: Adolescents demonstrated a significant positive automatic association of PA with social/appearance outcomes, F(1,136)=4.403, p<.05, η=.18, but they did not associate PA with health outcomes, F(1,136)=2.405, p=.123, η=.017. Multiple regression analyses found automatic social/appearance associations, explicit instrumental and affective attitudes, and the outcomes of enjoyment, stress management, challenge, competition, affiliation, and revitalization predicted PA in females only. Discussion: These results indicate that older adolescents automatically associate PA with desirable social/appearance outcomes, but not with health outcomes. Furthermore, the findings demonstrate the importance of considering both automatic associations and reasoned thought when assessing adolescent PA behaviour.