The reductive decomposition of calcium sulphate by hydrogen is used for the regeneration of calcium-based atmospheric fluidized bed combustion (AFBC) SO2 sorbents. The apparent solid¿solid reaction between CaS and CaSO4, one of the steps involved in the reaction mechanism of the reductive decomposition of CaSO4, has been investigated in a thermobalance construction provided with a coupled system for the analysis of product gases. By creating suitable reaction conditions in the experimental set-up, it is possible to study this step separately. The apparent solid¿solid reaction between CaS and CaSO4 takes place when a mixture of these compounds is heated in an inert atmosphere to temperatures above 1100 K: 3/4 CaSO4 + 1/4CaS - CaO + SO2 The fractional production rate of CaO (or SO2) appears to depend on the ratio in which CaSO4 and CaS are present initially. A simple model is formulated to calculate the fractional production rate as a function of the fractional degree of production and the initial composition of the reactants. One of the principal assumptions of the model is the presence of a liquid phase formed by mixtures of CaS and CaSO4. Fractional production rates calculated after determining the required model parameters (kinetic constants and volume plus composition of the liquid phase) show close agreement with experimentally obtained values.