The Clean Development Mechanism (CDM) of the Kyoto Protocol is supposed to provide both carbon mitigation and poverty reduction. This article reports from a model based study of market related carbon leakage and poverty reduction in the wake of a CDM tree-planting project in Tanzania. A tree plantation was incorporated in a computable general equilibrium (CGE) model with income differentiated household segments. The study focused on sensitivity of carbon leakage and income distribution to different project ownerships and carbon premium allocations. It turned out that the project value in terms of carbon premium has clear shortcomings as indicator of induced GDP growth and poverty alleviation. The non-poor rural and urban households benefit considerably more than the poor households. However, rising household income in all domestic project ownership arrangements increases demand for food, raises use of fertilizer and crop yields. A carbon cycle module for agricultural land use was incorporated in the CGE model, showing an increased carbon sequestration in agricultural soil, representing a negative leakage through markets in the range of 60–120% of the certified emissions reductions as registered in the CDM tree plantation project.