The effects of H2O and NH3 on the kinetics of the liquid phase hydrogenation of tetralin to decalin at 6.9 MPa and 330 degrees C over commercial P-Ni-Mo/alumina catalysts in the presence of H2S have been investigated. H2O functioned as a mild kinetic inhibitor to an extent sensitive to the H2S level. Quasi in situ XPS was used to investigate the catalyst structure after exposure to H2O/H2S.