One of the most contentious issues surrounding the forest conservation program REDD+ is the question whether it should be funded via international carbon markets. The controversy between market supporters and opponents has been especially marked in the public debate in Brazil, one of the main potential beneficiaries of REDD+ payments. In a remarkable shift of policy, the Brazilian Federal Government gave up its long-standing opposition to market-based funding in the run-up to the COP15, following several years of competition between two main discourse coalitions and their preferred story lines. These were analyzed here with discourse network analytical techniques. Brazil’s policy change may in part be explained by the failure of market opponents to employ positive arguments about alternative funding mechanisms, such as a public fund model; and by the increasing discursive dominance of a third emerging discourse coalition, which adopted major arguments of both sides in the debate. The research presented here thus provides more general insights on the dynamics of public debates, discourse coalitions, and the impacts of discursive strategies on policy-making, as well as on the value of discourse network analysis as a research method.