Catalysts containing 1, 3, and 8Vertical Bar3< molybdena on ..gamma..- or g-alumina were prepared by impregnating the support with (NH/sub 4/)/sub 6/Mo/sub 7/O/sub 24/ at pH 6 or 11, at which pH's molybdenum is in the octahedral Mo/sub 7/O/sub 24/(-6) form or the tetrahedral MoO/sub 4/(-2) form, respectively. The Raman and UV spectra indicated that adsorption on the alumina surface occurred in all cases via the MoO/sub 4/(-2) ion, probably due to locally increased pH. The removal of water during drying and calcination caused polymerization of the isolated tetrahedral molybdenum species into a two-dimensional Mo-O-Mo structure. The degree of polymerization depended on the molybdenum concentration, the heat treatment, and the pH of the original adsorption which influences the molybdenum distribution. The coordination number of the molybdenum in the polymeric structure, which was probably affected by the alumina surface hydroxyl density and distribution, could not be determined. Some bulk molybdenum trioxide was formed on all samples after calcination except for a 3Vertical Bar3< MoO/sub 3/ sample impregnated at pH 11.