The oxidation of CaS particles has been investigated by means of a thermogravimetric analyzer combined with a mass spectrometer, especially under fluidized bed combustion conditions. The products are CaSO4 or both CaSO4 and CaO, depending on the temperature and O-2 concentration. The oxidation reaction results in the evolution of SO2 when CaO is formed in the products. The conditions under which the undesirable SO2 will be released : or not, have been obtained. There is always an overall sample weight gain, even when SO2 is evolved during the reaction. The investigation of the kinetics of the CaS oxidation in the no SO2 releasing conditions shows that the oxidation reaction is of first order with respect to O-2, and a higher conversion can be achieved at higher temperature and by reducing the particle size. The maximum conversion of CaS to CaSO4 is 48.67% in 60 min at 865 degrees C and 4 vol% O-2 in N-2 for 17 mu m CaS particles. The rate constant and the effective product layer diffusivity are 4.1 x 10(-4)-2.88 x 10(-3) m s(-1) and 3.15 x 10(-11)-4.39 x 10(-10) m(2) s(-1) at 500-865 degrees C in the light of the shrinking unreacted core model for gas-solid reaction, the former yielding an activation energy of 39.5 kJ mol(-1) for the CaS oxidation reaction.