Limestone is decomposed by means of a thermogravimetric analyser (TGA). A special sample holder is designed in order to provide differential conditions. Three different size fractions are investigated isothermally under different CO2/N-2 mixtures. The reaction rate decreases exponentially with increasing CO2 partial pressure. An induction period is observed at low conversion. This can be explained by the evolution of the reacting surface area which is described by a modified random pore model. Both chemical kinetics and mass transport in the particle are reaction limiting factors. The amount, however, depends upon the particle texture, the initial particle size, and the CO2 partial pressure. A differential particle model is applied to a single particle. The model includes mass and heat transport, transport through a product layer and chemical kinetics. However, heat transport and product layer diffusion are of minor importance. Orthogonal collocation based upon finite elements is used to solve the model equations.