Batch formation of COS and H 2O by heterogeneous reaction of H 2S and CO 2 on A- and X-type zeolites with various degrees of alkaline earth cation-exchange, on faujasite-type zeolites with various Si Al ratios, and on alkaline cation-exchanged X-type zeolites, has been studied. Results obtained at different reaction times illustrate the dual function of the zeolites as selective sorbents causing a shift in reaction equilibrium and as catalysts. Correlations between the catalytic activity and population of cation sites with low oxygen coordination have been found. Furthermore, the catalytic activity could be correlated with the efficiency of dissociative H 2S adsorption (literature data) and the basicity of zeolites. These correlations are regarded as being in accordance with the concept of basic catalysis on zeolites.