Mixed feeding confers excess risk of mother-to-child transmission (MTCT) of Human Immunodeficiency Virus (HIV) compared with exclusive breastfeeding (EBF) and exclusive formula feeding (EFF). We undertook a quantitative and qualitative cross-sectional survey to identify the social determinants of mixed feeding among a subset of HIV-infected women enrolled in a MTCT prevention program in Jos, Nigeria. Of the 91 participants, 68(75%) exclusively formula fed, 7(8%) exclusively breastfed, and 16(18%) practiced mixed feeding. Of the mixed feeding women, 7 primarily formula fed and 9 primarily breastfed. Women who primarily formula fed described family pressure as the reason for mixed feeding, whereas women who primarily breastfed indicated insufficient breast milk. In a multivariate analysis, lack of partner support of the feeding decision predicted mixed feeding behavior (OR: 4.2; 95% CI: 1.2-14.9; p=0.03). Disclosure of HIV status was significantly correlated (p<0.001) with partner support. HIV prevention interventions aimed at reducing mixed feeding should encourage supportive partner relationships that facilitate disclosure of HIV status. Attention should also be made to the differing pressures faced by women attempting to exclusively breastfeed and exclusively formula feed.