Objectives: Physical activity is an important health behaviour especially for older adults. Forming implementation intentions is an effective strategy to implement physical activity in daily life for young and middle-aged adults.However, evidence for older adults is inconclusive. This study explored the thoughts of older adults about implementation intentions and potential barriers and facilitators while formulating them. Methods: Three samples of older adults from the United Kingdom (n = 8), Germany (n = 9) and Switzerland (n = 17) were prompted to think aloud while formulating implementation intentions to be more physically active. After the task, semi-structured interviews were conducted. Data were analysed thematically. Results: Participants expressed pre-established thoughts about implementation intentions (e.g. they feel too restrictive). During the formulation of implementation intentions, several barriers to creating them were reported (e.g. problems with finding cues due to absence of recurring daily routines), but participants also mentioned that forming implementation intentions acted as a facilitator for physical activity (e.g. cues as useful reminders to be active, task itself triggering self-reflection about physical activity). After the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.