One of the most striking features of molybdenum oxide is the versatility of its catalytic properties, which are determined by the valence state of molybdenum ions and their local environment. It may be anticipated that MoO3 surface must contain catalytic sites which are active in different types of elementary steps. Different probe catalytic reactions were thus applied to monitor the changes of the concentrations of various types of active sites in the course of the reduction of MoO3 to MoO2 and to unravel their role in the complex reaction network of such molecule as butene. Isomerization of 3,3-dimethylbutene-1 was used to characterize Bronsted acid sites, the ability of the MoO3 surface to generate allylic species was determined by the isomerization of 2,3-dimethylbutene-2. The presence of sites inserting oxygen into the hydrocarbon molecule was identified by the reaction of allyl iodide to form acrolein. Dehydrogenation of butene-1 to butadiene was applied as the test reaction to detect sites responsible for abstraction of beta hydrogen. Total oxidation was taken as a measure of the concentration of sites adsorbing electrophilic oxygen. The nature of different sites is discussed.