A calcined molybdenum catalyst prepared by impregnation of SiO2 with heptamolybdate has been doped with different amounts of K+ ions. The catalysts were characterized by X-ray absorption and Raman spectroscopy. Due to the lack of interaction of the polymolybdate species with the carrier in the Kf-free catalyst, MoO3 was formed after calcination. With increasing K+ content in the catalyst, the surface area of the SiO2 support drastically decreased and structural deformations occurred. The resulting molybdenum structures were well defined. In the uncalcined samples with ratios K/Mo less than or equal to 1, the molybdenum was 6-fold coordinated. At a K/Mo ratio of 2, the coordination changed into 4-fold and the samples contained mainly K2MoO4, which was differently distorted than in crystalline K2MoO4. Impregnating SiO2 with K2MoO4 led to spectra, which indicated a weak interaction of the Mo species with the SiO2 carrier.