What is known and objective:The recently conducted Medication Actions to Reduce hospital admissions through a collaboration between Community and Hospital pharmacists (MARCH) transitional care programme, which aimed to test the effectiveness of a transitional care programme on the occurrence of ADEs post-discharge, did not show a significant effect. To clarify whether this non-significant effect was due to poor implementation or due to ineffectiveness of the intervention as such, a process evaluation was conducted. The aim of the study was to gain more insight into the implementation fidelity of MARCH.Methods: A mixed methods design and the modified Conceptual Framework for Implementation Fidelity was used. For evaluation, the implementation fidelity and moderating factors of four key MARCH intervention components (teach-back, the pharmaceutical discharge letter, the post-discharge home-visit and the transitional medication review) were assessed. Quantitative data were collected during and after the intervention. Qualitative data were collected using semi-structured interviews with MARCH healthcare professionals (community pharmacists, clinical pharmacists, pharmacy assistants and pharmaceutical consultants) and analysed using thematic analysis.Results and Discussion: Not all key intervention components were implemented as intended. Teach-back was not always performed. Moreover, 63% of the pharmaceutical discharge letters, 35% of the post-discharge home-visits and 44% of the transitional medication reviews were not conducted within their planned time frames. Training sessions, structured manuals and protocols with detailed descriptions facilitated implementation. Intervention complexity, time constraints and the multidisciplinary coordination were identified as barriers for the implementation.