Prominent advocates of payments for ecosystem services (PES) contend that markets in biodiversity, carbon storage, and hydrological services can produce both conservation and sustainable development. In Mexico's national PES programs, however, conceived as models of market-based management, efficiency criteria have clashed with antipoverty goals and an enduring developmental-state legacy. Like other projects for commodification of nature, Mexico's PES is a hybrid of market-like mechanisms, state regulations, and subsidies. It has been further reshaped by social movements mobilized in opposition to neoliberal restructuring. These activists see ecosystem services as coproduced by nature and campesino communities. Rejecting the position of World Bank economists, they insist that the values of ecosystems derive less from the market prices of their services than from their contributions to peasant livelihoods, biodiversity, and social benefits that cannot be quantified or sold. These divergent conceptualizations reflect contrasting understandings of the roles of agriculture and of the state in sustainable development. The Mexican case exposes contradictions within neoliberal environmental discourse based on binary categories of nature and society. It suggests that conservation policies in the global South, if imposed from the North and framed by neoliberal logic, are likely to clash with state agendas and local development goals.