In Nigeria, most basic maternal and child health services in public primary health-care facilities should be either free of charge or subsidized. In practice, additional informal payments made in cash or in kind are common. We examined the nature, drivers and equity consequences of informal payments in primary health centres (PHC) in Enugu State. We used three interlinked qualitative methods: participant observation in six PHC facilities and two local government area (LGA) headquarters; in-depth interviews with frontline health workers (n = 19), managers (n = 4) and policy makers (n = 10); and focus group discussions (n = 2) with female service users. Data were analysed thematically using NVivo 12. Across all groups, informal payments were described as routine for immunization, deliveries, family planning consultations and birth certificate registration. Health workers, managers and policy makers identified limited supervision, insufficient financing of facilities, and lack of receipts for formal payments as enabling this practice. Informal payments were seen by managers and health workers as a mechanism to generate discretionary revenue to cover operational costs of the facility but, in practice, were frequently taken as extra income by health workers. Health workers rationalized informal payments as being of small value, and not a burden to users. However, informal payments were reported to be inequitable and exclusionary. Although they tended to be lower in rural PHCs than in wealthier urban facilities, participant observation revealed how, within a PHC, the lowest earners paid the same as others and were often left unattended if they failed to pay. Some female patients reported that extra payments excluded them from services, driving them to seek help from retail outlets or unlicensed health providers. As a result, informal payments reduced equity of access to essential services. Targeted policies are needed to improve financial risk protection for the poorest groups and address drivers of informal payments and unfairness in the health system.