Post-coital intravaginal cleansing (IVC) could counteract the protective effect of a vaginal microbicide. IVC less than 1 h after sex is discouraged in most microbicide trials. During a microbicide trial in KwaZulu-Natal, we collected quantitative data on post-coital IVC. We discussed IVC during in-depth-interviews (IDIs) and focus-group discussions (FGDs) with women enrolled in the trial, and during FGDs with community members. Nearly one-third (336/1,143) of women reported IVC less than an hour after sex. In multivariate analysis, post-coital IVC was associated with younger age, larger household size, greater sexual activity, consistent gel use, and clinic of enrolment. During IDIs and FGDs, respondents described post-coital IVC as a common hygiene practice motivated by the need to remove semen, vaginal fluids and sweat, although this practice may be amenable to change in the context of microbicide use. We need to consider strategies for influencing post-coital IVC practices in future microbicide trials and delivery programmes.