How does an adaptable goal-setting intervention change older adults’ self-determination and physical activity? A mixed-methods study Valérie Désirée Bösch and Jennifer Inauen University of Bern, Institute of Psychology, Department of Health Psychology and Behavioral Medicine Physical activity is an important health behavior especially for older adults. Postulated by the self-determination theory, one way to foster motivation to be physically active is by providing autonomy-supportive interventions, such as adaptive goals. However, this has not been investigated for older adults. We will examine whether older adults’ self-determination and physical activity changes during an adaptive goal intervention. Further, we will investigate what role self-determination plays when older adults talk about their physical activity, and how the intervention changes these perceptions. In a mixed-methods, within-person intervention study with intensive-longitudinal assessment (35 days) using diaries and accelerometry, participants (targeted N = 20) formulate a physical activity goal and corresponding implementation intentions. During the intervention period, participants will additionally get an autonomy-supportive intervention consisting of being provided with the opportunity to review and alter their goals and implementation intentions on a weekly basis. Before and after the intervention, a subsample of participants will be interviewed regarding their self-determination and physical activity. We expect that participants will improve their intrinsic motivation, intention to be active and their physical activity over time. The data collection is ongoing. We plan to analyze the quantitative data using multilevel modelling and the qualitative data by using combined thematic and interpretative phenomenological analysis. The study will firstly add an in-depth understanding about the role self-determination plays for older adults’ physical activity, how an intervention that is conceived to be autonomy-supportive is perceived and if this intervention can foster intrinsic motivation, intention and physical activity.