Abstract Objectives The COVID-19 pandemic has heightened the need to address loneliness and social isolation (and associated incidence of depression) amongst older adults. Between June and October 2020, the Behavioural Activation in Social IsoLation (BASIL) pilot study investigated the acceptability and feasibility of a remotely delivered brief psychological intervention (Behavioural Activation, BA) to prevent and reduce loneliness and depression in older people with long term conditions (LTCs) during the COVID-19 pandemic. Design An embedded qualitative study was conducted with semi-structured interviews to generate data that was first analysed inductively using thematic analysis and then deductively using the Theoretical Framework of Acceptability (TFA). Setting National Health Service and third sector organisations in England. Participants Sixteen older adults and 9 Support Workers (BSWs) participating in the BASIL pilot trial. Results Older adults and BSWs described a positive affective attitude towards the intervention linked to altruism, however the activity planning aspect of the intervention was limited due to COVID-19 restrictions. The intervention was understood by older adults & BSWs, although less understanding in older adults without low mood. A manageable burden was involved with delivering and participating in the intervention. For ethicality, older adults valued social contact and making changes, BSWs valued being able to observe those changes. Opportunity cost was low for BSWs & older adults. BA was perceived to be useful in the pandemic and likely to achieve its aims, (Perceived Effectiveness) especially if tailored to people with both low mood and LTCs. Self-efficacy developed over time and with experience for both BSWs and older adults. Conclusions Overall, the BASIL pilot study processes and BA intervention were found to be acceptable. Use of the TFA provided valuable insights into how the intervention was experienced and how the acceptability of study processes and the BA intervention could be enhanced ahead of the larger definitive trial (BASIL+). Strengths & Limitations The use of TFA in both informing the topic guide and conducting the analysis, demonstrating a systematic enquiry into acceptability, and contributing to the wider field as well as the topic area. The length of the interviews facilitated an in-depth exploration of older adults and BASIL Support Workers’ experiences. Conducting the interviews by telephone whilst discussing feasibility of telephone delivery may have enabled contextual cues to be discussed that may have been missed in a face-to-face interview set up, however may have led to a self-selecting sample of people who were comfortable with the telephone. A limitation is that the short timescale for the study meant that participants had to be interviewed as they completed 3m outcome measures, rather than using strategic sampling.