The United Nations negotiations on climate change have focused their attention on a set of policies for Reducing Emissions from Deforestation and Degradation (REDD+). This paper explores the potential of REDD+ to reduce CO2 emissions and protect tropical biodiversity. The study uses ArcGIS to model forest areas under threat of deforestation in 59 tropical developing countries. A constrained linear optimization model, implemented with linear optimization software, is used to construct a Conservation Possibilities Frontier (CPF). The CPF shows the potential of REDD+ to achieve emission reductions and species conservation under limited budgets. I use linear optimization to construct marginal abatement cost curves under various policy scenarios and estimate the costs of generating biodiversity co-benefits from REDD+. An international mechanism mainly designed to reduce emissions at least cost will provide low conservation benefits. Incorporating provisions for biodiversity co-benefits in the REDD+ framework can protect a high number of rare and threatened forest species at a relatively low cost.