Background <p>Bereavement-related grief and prolonged grief disorders are highly prevalent; however, standardised care approaches are lacking. To address this gap, the self-COMPAssion-based online group psyChoTherapy for bereavement-related grief (COMPACT) feasibility trial was developed; it showed promising participant engagement and potential effectiveness. This study aimed to elucidate the mechanisms underlying the impact, contextual factors, and implementation considerations of the COMPACT programme.</p> Methods <p>Online semi-structured interviews were conducted and analysed using a deductive reflexive thematic analysis guided by the UK Medical Research Council process evaluation framework. The interviews involved 21 participants and 10 intervention personnel from the COMPACT feasibility trial. The Helpful Aspects of Therapy Form (HATF) was used to guide the interviews, ensuring a focus on identifying mechanisms of impact, contextual factors, and implementation considerations.</p> Results <p>Four main “mechanism of impact” themes were identified: common humanity and self-awareness, perceived importance of self-care, deepening self-insights and personal growth, and amplified self-compassion. Two associated “contextual factor” themes (group therapy and a secure programme environment) were highlighted. Additionally, two “implementation consideration” themes (barriers and facilitators) were found, with related contextual themes of group therapy and online delivery.</p> Conclusions <p>The core impact mechanisms of the COMPACT programme included accessible online delivery, group work enhancing common humanity, and a safe, supportive environment deepening self-insight, self-care, and self-compassion. Future research should prioritise refining programme content, improving participant assessment, and enhancing training for intervention personnel to enable a randomised controlled trial testing the effectiveness of the intervention.</p> Trial registration <p>UMIN000048554, registered 2nd August 2022.</p>