The uptake of S-35-labelled H2S, the thiophene hydrodesulfurization (HDS) activity in pulse and flow system and the acidity measured by infrared (LR) band intensities of adsorbed pyridine were studied after various treatments over NiW/amorphous silica-alumina (ASA), NIW/Al2O3 and NiMo/Al2O3 catalysts. The Fourier Transformation Infrared (FTIR) measurements indicated that the NiW/ASA catalyst contained some Bronsted and much more Lewis acid sites, whereas less Bronsted acid sites were observed on the alumina supported samples. The concentration of Bronsted and Lewis acid sites decreased after sulfidation. The amount of total sulfur uptake of NiMo/Al2O3 catalyst was about twofold of the one of NiW samples. The sulfidation level of the catalysts was much lower in the pulse system than in the flow one and also the thiophene conversion was much lower on NiW catalysts in the former setup than in the latter system. A band was detected in the radiochromatogram of NiW/ASA catalyst during desorption of H2S in the pulse system which was attributed to weakly adsorbed H2S on the Bronsted acidic sites. This weakly adsorbed H2S hindered the isomerization of n-butane in the flow system but not in the pulse system.