Few would disagree that 'social context' shapes the effectiveness of HIV-prevention programmes. However much work remains to be done in developing systematic conceptualisations of HIV/AIDS-relevant aspects of social environments in vulnerable communities. This paper contributes to this challenge through a case study (44 interviews, 11 focus groups with 55 people and fieldworker diaries) of the impact of social context on a participatory peer education programme involving young people in a peri-urban community in South Africa. Three interacting dimensions of context undermine the likelihood of effective HIV-prevention. Symbolic context includes stigma, the pathologisation of youth sexuality (especially that of girls) and negative images of young people. Organisational/network context includes patchy networking amongst NGOs, health, welfare and education representatives and local community leaders and groups. This is exacerbated by different understandings of the causes of HIV/AIDS and how to manage it. These challenges are exacerbated in a material-political context of poverty, unemployment and crime, coupled with the exclusion of young people from local and national decision-making and politics. HIV-prevention initiatives seeking to promote health-supporting social environments should work closely with social development programmes to promote young peoples' social and political participation, increase opportunities for their economic empowerment, challenge negative social representations of youth, and fight for greater recognition of their sexuality and their right to protect their sexual health.