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Background: Research investigating the psychological determinants of physical activity (PA) tends to conceptualize determinants as stable, trait-like factors. Growing evidence suggests that people and the processes that regulate people's behavior fluctuate over short timescales (eg, hours and days). This study explores the extent to which subject-level variability (vs stability) in determinants of PA, as well as the interaction between subject-level mean and variability in those determinants, is associated with moderate- to vigorous-intensity PA (MVPA) in older adults., Methods: Older adults (N = 202) enrolled in a measurement burst design study with 3 data collection waves occurring over a 1-year period. Each data collection wave consisted of accelerometers and ecological momentary assessment to capture MVPA and psychological determinants of MVPA, respectively. Mixed-effects location scale modeling was first used to estimate subject-level means and variability in psychological determinants. These estimates were then used as predictors of daily MVPA using multilevel modeling., Results: Subject-level variability in PA intentions (β = 0.199, SE = 0.062, P = .001), self-efficacy (β = 0.133, SE = 0.064, P = .037), and planning (β = 0.154, SE = 0.062, P = .014) were positively associated with daily MVPA. Associations between subject-level variability in PA intentions and self-efficacy decreased in strength, becoming nonsignificant at higher mean levels of PA intentions (β = -0.147, SE = 0.062, P = .018) and self-efficacy (β = -0.116, SE = 0.063, P = .062)., Conclusions: Greater subject-level variability in psychological determinants may be adaptive for PA engagement, especially among older adults experiencing low levels of those determinants. Moving beyond mean levels to include subject-level variability in psychological determinants of PA may help to improve our understanding of the processes that drive PA engagement.