Reversible transformations Ni(IV) <-> Ni(II) in alumina and Sibunit supported (Ni,Mo) sulfide catalysts were observed after in situ thermal treatment of catalysts in an X-ray photoelectron spectrometer chamber. The phenomenon is interpreted as a reductive elimination of occluded hydrogen under low pressure and high temperature, and oxidative addition of hydrogen after catalyst treatment with an (H-2 + H2S) mixture.