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ChemInform Abstract The surface structures of calcined and reduced Mo/Al2O3 catalysts are studied by Fourier-transform IR spectroscopy, TGA reduction studies, and CO2 and NO chemisorption. There are three different surface structures depending on the Mo loading. At low loadings, Mo interacts strongly with hydroxyl groups of the support. The Mo is highly dispersed, difficult to reduce, and has a low coordination number, CN, probably tetrahedral symmetry. At medium loadings (up to monolayer coverage), the Mo-O-Mo interaction increases and the Mo-support interaction may decrease. This Mo is well dispersed, reducible, and has a high CN, probably octahedral symmetry. Above monolayer coverage, excess Mo aggregates to form bulk MoO3 and Al2(MoO4)3 species at the expense of the well-dispersed species. The Mo is easily reduced, has a large particle size and a high CN. All the Mo phases rearrange and agglomerate to create some free alumina after H2 reduction.