The scientific information to support the causal relationship between input/output and outcome/impact of HIV prevention is inadequate and fragmented. A properly conducted economic evaluation could present reasoned and justifiable arguments as to why more or fewer resources should be directed towards particular HIV prevention interventions. However, current economic evaluations have attempted to establish a mechanical relationship between a specific intervention and the outcome in individuals. None of the current economic evaluations have been adequate to capture the relationship between inputs and the increase in social capital resulting from these interventions. Therefore, there is a need for developing a more holistic framework for economic evaluations assessing interventions with complex, interrelated social outcomes. This report responds to the request of the Asian Development Bank to conduct an economic evaluation on the AIDS Competence Process (ACP). This assessment aims to (i) develop a methodological framework for assessing the cost-effectiveness of the ACP and applying such a framework retrospectively to implementation in Asia and the Pacific; and (ii) provide capacity building recommendations to the Constellation to routinely measure and improve cost-effectiveness in its programmes, applying the methodological framework. This study was done with some constraints, as the ACP was not designed for economic appraisal and cost and outcome data appropriate for comprehensive cost-effectiveness or cost-utility analysis were lacking. Thus, this study rather aims to explore the information gap for future economic evaluation of the ACP, to guide the ACP’s surveillance and monitoring system as well as to demonstrate the application of a newly developed methodological framework that could be applied for future evaluation of other social complex interventions.In comparing costs and outcomes of the ACP using various scenarios, it is found that the ACP is likely to be very cost-effective in Thailand. The ACP saves one QALY using resources valued less than 1 Gross Domestic Product per capita (approximately 140,000 Baht), which is a cost-effectiveness benchmark defined by the National Health Security Office who manages the HIV prevention program in Thailand. The ACP is a cost-saving intervention if it increases condom use in FSWs or MSM by more than 10%. In addition, this assessment suggests that the higher the number of population reached by the ACP, the more cost-effective the results are, as does not only the ACP reduce HIV risk behaviours resulting in HIV infections averted, but also improves individual capability resulting in increased quality of life among individuals. Although the evaluation shows that the ACP is likely to be very cost-effective in Thailand and other developing country settings, future research is needed to give reliable information regarding the intervention effectiveness, especially in relation to its ultimate goals, for example, number of HIV infections averted or QALYs gained. This report recommends actions on how to improve monitoring and evaluation, and possible tools for economic assessments for future design of new programmes supported by the Constellation.