Decomposition of CaS particles (18.6 mu m size) at ambient conditions was investigated by means of potentiometric analysis, TG, SEM-EDX and XRD techniques. The thermodynamics of the process was also studied. The results show that well dispersed CaS particles are decomposed provoking a high H2S release to the atmosphere during the first two days of decomposition. This release was evaluated to be in the range 61-72 mol% on initial sulphur basis. The remaining solid consists of a 66 mol% calcium carbonate, 22 mol% calcium sulphate and 12 mol% elemental sulphur. Trace amounts of thiosulphate were also detected by XRD analysis. The formation mechanisms of the different species are discussed. SEM photographs of decomposed CaS pellets revealed that the decomposition process is diffusion controlled for high loads of particles.