Costs of reforestation projects determine their competitiveness with alternative measures to mitigate rising atmospheric CO₂ concentrations. We quantify carbon sequestration in above-ground biomass and soils of plantation forests and secondary forests in two countries in South America—Ecuador and Argentina—and calculate costs of temporary carbon sequestration. Costs per temporary certified emission reduction unit vary between 0.1 and 2.7 USD Mg⁻¹ CO₂ and mainly depend on opportunity costs, site suitability, discount rates, and certification costs. In Ecuador, secondary forests are a feasible and cost-efficient alternative, whereas in Argentina reforestation on highly suitable land is relatively cheap. Our results can be used to design cost-effective sink projects and to negotiate fair carbon prices for landowners.