Background: Communication between people with aphasia and their healthcare professionals (HCPs) can be greatly improved when HCPs are trained in using supportive conversation techniques and tools. Communication partner training (CPT) is an umbrella term that covers a range of interventions that train the conversation partners of people with aphasia. Several CPT interventions for HCPs have been developed and used to support HCPs to interact successfully with people with aphasia. Aims: The objective of this study was to identify the mechanisms of change as a result of a Dutch CPT intervention, named CommuniCare, in order to evaluate and optimise the intervention. Methods & procedures: A total of 254 HCPs from five different healthcare centres received CommuniCare. An explorative qualitative research design was chosen. Two interviews were conducted with 24 HCPs directly after and 4 months after receiving the training that was part of CommuniCare. Two conceptual frameworks were used to deductively code the interviews. HCPs' perspectives were coded into a four-part sequence following CIMO logic: the self-reported use of supportive conversation techniques or tools pre-intervention (Context), the intervention elements (Intervention) that evoked certain mechanisms (Mechanisms), resulting in the self-reported use of supportive conversation techniques and tools post-intervention (Outcomes). The Capabilities Opportunities Motivation-Behaviour (COM-B) model was used to fill in the Mechanisms component. Outcomes & results: Three themes were identified to describe the mechanisms of change that led to an increase in the use of supportive conversation techniques and tools. According to HCPs: (1) information, videos, e-learning modules, role-play, feedback during training and coaching on the job increased their psychological capabilities; (2) information and role-play increased their automatic motivations; and (3) information, videos and role-play increased their reflective motivations. Remaining findings show HCPs' perspectives on various barriers to use supportive conversation techniques and tools. Conclusions & implications: HCPs in this study identified elements in our CPT intervention that positively influenced their behaviour change. Of these, role-play and coaching on the job were particularly important. HCPs suggested this last element should be better implemented. Therefore, healthcare settings wishing to enhance HCPs' communication skills should first consider enhancing HCPs' opportunities for experiential learning. Second, healthcare settings should determine which HCPs are suitable to have a role as implementation support practitioners, to support their colleagues in the use of supportive conversation techniques and tools.