Effective youth participation in social development and civic life can enhance young peoples' health and well-being. Yet many obstacles stand in the way of such involvement. Drawing on 105 interviews, 52 focus groups and fieldworker diaries, this paper reports on a study of a rural South African project which sought to promote effective youth participation in HIV/AIDS management. The paper highlights three major obstacles which might be tackled more explicitly in future projects: (i) reluctance by community adults to recognise the potential value of youth inputs, and an unwillingness to regard youth as equals in project structures; (ii) lack of support for meaningful youth participation by external health and welfare agencies involved in the project; and (iii) the failure of the project to provide meaningful incentives to encourage youth involvement. The paper highlights five psycho-social preconditions for participation in AIDS projects (knowledge, social spaces for critical thinking, a sense of ownership, confidence and appropriate bridging relationships). We believe this framework provides a useful and generalisable way of conceptualising the preconditions for effective "participatory competence" in youth projects beyond the specialist HIV/AIDS arena. (Contains 4 notes.)