Exclusive breastfeeding (EBF) has the potential to significantly reduce infant mortality, but is frequently not practiced in low-income settings where infants are vulnerable to malnutrition and infections including human immunodeficiency virus (HIV). This study explores mothers' experiences of infant feeding after receiving peer counselling promoting exclusive breast or formula feeding. This qualitative study was embedded in a cluster randomized peer counselling intervention trial in South Africa that aimed to evaluate the effect of peer counselling on EBF. Participants were selected from the three districts that were part of the trial reflecting different socio-economic conditions, rural-urban locations and HIV prevalence rates. Seventeen HIV-positive and -negative mothers allocated to intervention clusters were recruited. Despite perceived health and economic benefits of breastfeeding, several barriers to EBF remained, which contributed to a preference for mixed feeding. The understanding of the promotional message of 'exclusive' feeding was limited to 'not mixing two milks': breast or formula and did not address early introduction of foods and other liquids. Further, a crying infant or an infant who did not sleep at night were given as strong reasons for introducing semi-solid foods as early as 1 month. In addition, the need to adhere to the cultural practice of 'cleansing' and the knowledge that this practice is not compatible with EBF appeared to promote the decision to formula feed in HIV-positive mothers. Efforts to reduce barriers to EBF need to be intensified and further take into account the strong cultural beliefs that promote mixed feeding.