Abstract Background General practice is often recommended as an ideal setting to initiate advance care planning (ACP), but uptake of ACP in this setting is low. ACP-GP is a complex intervention to facilitate ACP for patients with chronic, life-limiting illness in Belgian general practice. It aims to increase patient ACP engagement and general practitioner (GP) ACP self-efficacy. In a cluster-randomized controlled trial, the intervention was not superior to control in increasing these outcomes. A parallel process evaluation aimed to enhance understanding of how the intervention was implemented, and which factors might have influenced trial results. Methods We conducted a mixed-methods process evaluation following the Reach, Effectiveness, Adoption, Implementation, Maintenance (RE-AIM) framework. Data sources include recruitment and implementation monitoring, questionnaires for patients and GPs, and semi-structured (focus group) interviews with patients and GPs. Questionnaire data were analyzed descriptively. ...