A series of oxidic MoO3/Al2O3 catalysts with different Mo loading was prepared by reaction of alumina (172 m(2)/g) with aqueous slurry of MoO3 and by conventional impregnation with ammonium heptamolybdate. Surface concentrations of OH groups on the samples have been determined as a function of pretreatment temperature (160-500 degrees C) and loading (up to 19.3% MoO3) by titration with dimethylzinc reagent. It was found that consumption of OH groups of alumina by MoO3 proceeds during catalyst calcination and is completed already at 315 degrees C. Further thermal treatment led only to dehydroxylation of alumina support. The gradual consumption of OH groups by MoO3 can be monitored up to saturation loading 14.4% MoO3. It was described by a linear decrease of the number of OH groups up to about 7.8% MoO3, followed by a nonlinear dependence between 7.8-14.4% MoO3. An expression describing the change of OH/Mo stoichiometry (OH consumed per Mo atom) with MoO3 loading was derived. The OH/Mo value decreased from about 2 for the lowest MoO3 loadings to about 0.8, close to 14.4% MoO3. Above this loading, the content of OH groups decreased to 20-25% of the original value and remained constant irrespective of the excess of MoO3 added. As a consequence, the OH/Mo stoichiometry approached zero. The residual hydrogen is ascribed mainly to the nonreactive hydroxyls of alumina and probably to a small extent to the Mo-OH groups originating from MoO3.