The private sector provides a large proportion of health services in many low- and middle-income countries (LMICs), particularly for primary health care (PHC), even for poor patients. But the role of the private sector in expanding universal health coverage (UHC) in LMICs remains contentious. Proponents argue that the private sector could increase patient choice and PHC access, and that competition incentivises private providers to be more responsive and provide better quality care than public providers. However, evidence suggests that these advantages may not materialise. Arguments supporting the role of the private sector rely on the assumption that private providers compete for patients. Yet many factors influence market performance and health markets are often not competitive. Effective competition also requires that patients respond to changes in price or quality. Although these dynamics are critical in determining the health system impacts of private PHC provision in LMICs, they have received little attention in the literature. Understanding the role and impact of private sector provision is especially important in South Africa as policy proposals for achieving UHC promote the contracting of private providers to expand access to quality PHC for uninsured patients. There are concerns about the current performance of private PHC providers, and the functioning of the PHC market, with little empirical evidence to inform current debates. Expanding the role of the private sector as part of efforts to achieve UHC requires a more thorough understanding of the potential risks and benefits, and the likely responses of both the supply and demand sides of the market. The aim of this study is to undertake a detailed empirical investigation of the market for public and private primary care services. It will focus on the determinants of provider performance on the one hand, and demand for private services from uninsured cash-paying patients on the other. The study will be conducted in Soweto, Johannesburg, and it will include five components. Firstly, we will undertake a detailed description of the local PHC market through a census, mapping and interviews of all providers, an analysis of market concentration, and investigation of the strategies which private providers use to compete for patients. Secondly, using 'fake' standardised patients (SPs), we will compare the performance of private and public providers in terms of accessibility to services, technical quality of care and cost of treatment recommended. Thirdly, we will establish the relationship between competition and performance outcomes, testing if greater competition leads to better outcomes. Fourthly, using linked data on provider performance and cost, we will investigate if accessibility, quality and cost are important determinants of the demand for services by uninsured patients. Finally, in a small randomised pilot, we will test study how populations would react to the introduction of subsidised access to private services, and explore if information about quality influences demand. The study will provide important information on whether the private PHC market can contribute to better health system access, quality and efficiency. The results are relevant to many LMICs trying to expand UHC within mixed health care systems.