For older adults, physical activity is an important health behaviour. Implementation intentions are an effective strategy to implement physical activity in daily life. However, evidence for older adults is inconclusive. This study explored potential difficulties and facilitators older adults face when formulating implementation intentions and their related beliefs. 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. After the task, semi-structured interviews were conducted. Data were analysed using thematic analysis. The data suggested that older adults can have pre-established negative beliefs about planning before formulating implementation intentions (e.g. that planning is too restrictive). During the formulation of implementation intentions, participants reported several barriers (e.g. absence of a recurring daily routine) and facilitators (e.g. that the cue was a useful reminder to be active, and that the task triggered self-reflection about physical activity). After the task, a caveat for using implementation intentions mentioned was that they are not always applicable due to several circumstances, e.g. spontaneous alternative activities, weather, health-related or Covid-19-related barriers. In the Swiss sample, forming implementation intentions spontaneously triggered coping planning. The results on barriers and facilitators of implementation intentions from older adults’ perspectives provide starting points for improving implementation intention effectiveness related to physical activity in this population. Which could lead to better tailored interventions using implementation intentions for physical activity enhancement in older adults in the future.