The influence of temperature and oxygen partial pressure on the oxidation of pure CaS particles (empty set = 18.6 mu m) in the regime of low SO2 release (T < 890 degrees C for p(O2,O) = 0.21 atm, or T < 840 degrees C at any oxygen partial pressure) has been studied by means of thermogravimetric analysis. The kinetics of the process was analysed with the unreacted core model. After a short period of chemical control, the process is governed by solid-state diffusion through the product layer. The effective diffusivity of oxygen in the CaSO4 lattice decreases with conversion and is a function of temperature and oxygen partial pressure. This dependence can be explained by a grain boundary diffusion mechanism. This is thought to proceed via migration of Ca2+ and S2- ions from the CaS/CaSO4 interface to the particle surface, where the ions are oxidised to CaSO4 by gaseous oxygen.