COS adsorption was studied by infrared spectroscopy on gamma-alumina activated at various temperatures. Evidence for hydrogen thiocarbonate (HTC) species was provided from the interaction with monodentate OH groups, the most basic. On highly activated samples, additional weakly chemisorbed species occurred through the interaction between COS and Lewis acid or basic sites. HTC species were shown to play the role of intermediates in the COS hydrolysis reaction. Conversely, studies at low temperature showed that hydrogen carbonate species resulted from the readsorption of CO2, a reaction product. Quantum chemistry calculations corroborated the infrared results and provided complementary information regarding the hydrolysis mechanism. In particular, the cyclic geometry of the transition state was found to occur through a nearly regular hexagonal configuration. Moreover, the activation barriers for HTC formation and hydrolysis were calculated and found to be similar.