BACKGROUND: Age-disparate sex is associated with increased HIV risk among adolescent girls and young women (AGYW) in sub-Saharan Africa. However, little has been done to understand the dynamics of such relationships from the perspectives of either AGYW or older men, and the communities in which these relationships are embedded. This article explores the motivations and perceived benefits of such relationships for AGYW and older men, plus the social and sexual and reproductive health (SRH) consequences. METHODS: This study held 37 participatory focus group discussions and 87 in-depth interviews with young people aged 14-24 and adult community members aged 25-49 in rural and urban Tanzania and Uganda. Participants were sampled using purposive and snowball techniques. Thematic analysis was conducted with the aid of NVIVO 10 software. RESULTS: Motivations, perceived benefits and costs for AGYW centred around four main themes: financial motivations, emotional support, meeting social expectations and reflections on sexual health. Specifically, AGYW noted that older partners gave gifts/money of higher value compared with younger men. Men's perceived benefits and costs revolved around the need to satisfy their sexual desire, the perception that AGYW were capable of engaging in new and creative sexual styles and their desire for prestige among male peers. Both AGYW and men recognised the social and SRH consequences as: risk of violence, social stigma, risk of unplanned pregnancy and risk of sexually transmitted infections including HIV. CONCLUSION: Interventions need to acknowledge the perceived benefits of age-disparate sexual relationships for AGYW and older men and engage them in critical reflection on the medium- to longer-term consequences versus the shorter-term satisfaction of needs, desires and aspirations, as a way to navigate the constrained opportunities they face given existing structural limitations. Interventions should also tackle the structural constraints AGYW face by helping them access resources, become empowered and challenge the expectation of having to depend financially on men. Interventions with men should unpack the assumption that men are naturally hypersexual. The role of peers for both girls and men should be acknowledged, and a shift from individual targeted interventions to changing norms at the community level should be considered.