Background: Improving our understanding of modifiable factors related to older adults' moderate-to-vigorous physical activity (MVPA) behaviour may help to inform behaviour change interventions. How older adults experience MVPA, and whether this influences the source of their motivation, may be viable targets. Purpose: To examine if changes in affect during a bout of MVPA were directly and indirectly (via self-determined motivation) associated with future MVPA behaviour older adults. Methods: Data were collected from 82 older adults (Mage=68.43 years) five minutes before, immediately before, during, and five minutes after walking on a treadmill at moderate-to-vigorous intensity for 30 minutes. One month later, participants' MVPA behaviour was assessed during a phone call. Changes in affect were computed by subtracting mean scores before the bout from mean scores during the bout. Data were analyzed via SPSS mediation macro with bootstrap estimation procedures. Results: Adjusting for baseline MVPA, sex, and body mass index, changes in revitalization, positive engagement, and tranquility were directly and positively associated with MVPA (ps<.05). Self-determined motivation was directly and positively related to MVPA (p<.05). Changes in tranquility were indirectly associated with MVPA via self-determined motivation (p<.05). Conclusions: Although the literature supports that MVPA improves affect, this study adds to the evidence that affective responses during MVPA can also influence future MVPA participation. Further, the findings offer a possible explanation for how affective responses may come to influence MVPA. Older adults' affective responses to MVPA should be considered in motivationally-tailored strategies to foster MVPA adherence.