Carbonyl-precursor-based W/Al2O3 and bimetallic CoW/Al2O3 catalysts were prepared by gas-phase adsorption in a fluidized-bed reactor. The surface species formed during the gradual and controlled preparation process were studied by temperature-programmed methods. Interactions on the surface were investigated as a function of metal loading by temperature-programmed oxidation (TPO) and oxygen pulse chemisorption (PCO). A clear relationship was observed between decarbonylation treatment and the tungsten species formed. Total acidity of the samples was determined by temperature-programmed desorption of ammonia (NH3-TPD). The NH3-TPD measurements, together with previous activity studies, suggest a relationship between total acidity and hydrotreating activity. The results of PCO and NH3-TPD measurements indicate that when the controlled gas-phase preparation method is applied to zerovalent carbonyl precursor, the unfavourable formation of tungsten oxide can be minimized.