Gender norms that privilege men's sexual power and pleasure, and distrust of condom use in intimate relationships, leave women vulnerable to HIV and other sexually transmitted infections. Vaginal microbicides allow women to exert a degree of control over their sexual health, through responsibility for product insertion as well as the possibility of covert use. In practice, however, the uptake of new HIV-prevention products is heavily influenced by partnership dynamics. This paper presents a secondary analysis of data from two qualitative sub-studies conducted during a Phase 3 microbicide efficacy trial in South Africa. Using transcripts from in-depth interviews and focus group discussions with 278 female trial participants and 27 male partners, we investigated the extent to which women disclosed microbicide use to their partners, and the level and types of male engagement with microbicide use. Most women chose to communicate with their partners about the trial, but the timing and content of associated discussions differed according to their motivation for disclosure. Men provided their partners with both moral and practical support, but reported a desire for greater involvement in decision-making surrounding microbicide uptake and use. The findings inform recommendations for constructive male participation in future trials and, ultimately, introduction of a marketed product.