Calcined molybdenum-containing catalysts, prepared by impregnation of Al2O3 with heptamolybdate, were doped with different amounts of K+ ions. Two other catalysts with a K/Mo ratio of 2 were prepared by impregnation of Al2O3 with K2MoO4. The Mo surface structures of these catalysts were characterized by X-ray absorption and Raman spectroscopy. The undoped samples contained the molybdenum in a 6-fold coordination as polymolybdate. With increasing K+ content, these polymolybdate species depolymerized progressively. At the K/Mo ratio of 2, most of the Mo ions were in a 4-fold coordination. The samples mainly contained K2MoO4 differently distorted than in crystalline K2MoO4, probably due to an interaction with the alumina support. The composition of the samples seemed to determine predominantly the nature of the Mo species stabilized on the alumina in the catalysts with K/Mo = 2.