As part of a larger study to examine the feasibility and acceptability of a couples-based HIV-prevention intervention, we conducted formative in-depth interviews with 10 couples to explore topics such as challenges in practising safer sex, HIV-prevention strategies, gender power and violence, and issues of trust and infidelity. In this study, both men and women perceived infidelity as ubiquitous in their social context and were therefore unable to discuss HIV risk and prevention without suspicions of infidelity in their own relationship. This impacted couples' ability openly and effectively to discuss strategies to prevent HIV and thus may have contributed to the limited uptake of HIV-prevention strategies, such as condom use and HIV testing. The contentious nature of safe-sex discussions placed both members of the couple at a higher risk for HIV acquisition within the partnership. This study sheds light on how existing relationship norms in South Africa influence HIV-prevention communication within couples and suggests that new ways of approaching conflictual issues such as mistrust and infidelity are vital in order for HIV-prevention programmes to succeed.