This paper reports the process evaluation of a peer group intervention for human immunodeficiency virus (HIV) prevention which had positive outcomes for three target groups in Malawi: rural adults, adolescents and urban hospital workers. The six-session intervention was delivered to small groups of 10-12 participants by 85 trained volunteer peer leaders working in pairs. A descriptive, observational mixed methods design was used with a convenience sample of 294 intervention sessions. Using project records and a conceptually based observation guide, we examined five aspects of the implementation process. The context was favorable, but privacy to discuss sensitive issues was a concern for some groups. In study communities, program reach was 58% of rural adults, 70% of adolescents and nearly all hospital workers. Session records confirmed that all peer groups received the intended six sessions (dose delivered). The dose received was high, as evidenced by high participant engagement in peer group activities. Peer leaders were rated above the median for three indicators of peer group content and process fidelity: session management skills, interpersonal facilitation skills and whether more like a peer group than classroom. Documenting that this HIV prevention peer group intervention was delivered as intended by trained peer volunteers supports widespread dissemination of the intervention. (Contains 7 tables.)