This paper describes the plan for a process evaluation of a quasi-experimental study testing the municipal level scale-up of primary health care-based measurement and brief advice programmes to reduce heavy drinking and comorbid depression in Colombia, Mexico, and Peru. The main aims of the evaluation are to assess the implementation of intervention components; mechanisms of impact that influenced the outcomes; and characteristics of the context that influenced implementation and outcomes. Based on this information, common drivers of successful outcomes will be identified. A range of data collection methods will be used: questionnaires; interviews; observations; logbooks; and document analysis. All participating providers will complete a pen-and-paper questionnaire at recruitment and two time points during the implementation period. Providers attending training will complete post-training questionnaires. Additionally, 1080 patients will be invited to self-complete a patient questionnaire. One-in-ten participating providers and fifteen other key stakeholders will participate in semi-structured interviews. Training sessions and community advisory board meetings will be observed by a neutral observer. Logbooks will be kept by local research teams to document events affecting the implementation. Project related documentation and other relevant reports describing the context will be examined.