The relationship between H2S emissions from three-way catalysts and the storage of sulphur on the catalyst surface has been investigated.Thermodynamic data predict that sulphur storage primarily will occur on Al2O3 and CeO2 under lean and stoichiometric conditions, at up to 500°C. Rich transients could then induce the decomposition of the Ce-S-O and Al-S-O compounds, releasing sulphur into the gas phase. Experimental studies have supported this model. A mechanism has been proposed for the subsequent formation of H2S.The mechanism by which catalyst poisons attenuate H2S emissions from engine-aged catalysts also has been studied. The effect has been shown to be related to decreased storage of sulphur, caused by stable catalyst-poison species at the catalyst surface.