Quantitative results for the coverage of surface hydroxyl groups on eta-Al2O3 and calcined molybdenum catalysts supported on Al2O3 have been obtained by H-1 MAS NMR spectroscopy. By studying catalysts with different molybdenum concentrations, information on the nature of the reaction of molybdate ions with the structurally different hydroxyl species of the support could be obtained. The present studies show that a preferential interaction of molybdenum with the most basic hydroxyl groups of eta-Al2O3 takes place and that monolayer structures dominate up to approximately 12 wt% Mo corresponding to 4.0 Mo atoms per nm(2).