Oxidation experiments of pure CaS particles (18.6 mu m size) in the regime of high SO2 release (890 degrees C < T < 1175 degrees C at P-O2,P-O = 0.21 atm) were carried out in a thermogravimetric analyser. Additionally, the evolution of SO2 liberation during oxidation was followed by experiments carried out in a quartz microreactor. SEM-EDX analyses of oxidised CaS pellets allowed the structural changes during oxidation to be disclosed. Air oxidation of the CaS particles used in this work, consisting of agglomerates of CaS crystallites, takes place via oxidation of the CaS surfaces to CaSO4. The product so formed rapidly reacts with the contiguous CaS via a solid-solid reaction to form small CaO grains of a rounded shape. These grains are sulphated by the SO2 released in the solid-solid reaction, thus growing in size and eventually plugging the pore structure of the particle. The growing of the CaO grains involves crystallisation towards a cubic-like shaped CaSO4 phase. A fraction of the SO2 formed via the solid-solid reaction is released to the bulk gas phase during the first stage of reaction, before closure of the pore network of the particles takes place. Afterwards, the gases diffuse inside the particles via a solid-state mechanism. A kinetic model for CaS oxidation, which takes into account the above consideration, was developed.