Al2O3- and SiO2-supported oxidic molybdenum catalysts with different Mo and K contents were prepared by subsequent impregnation with an aqueous heptamolybdate solution and a K2CO3 solution. Samples were also prepared by impregnation with K2MoO4. The reducibility of the Mo compounds was related to tl;e oxidic molybdenum structures stabilized on the carrier surface. Furthermore, TPR results were interpreted in terms of surface interaction between molybdenum compounds and the alumina surface, having a wide variation of bond strengths. Thiophene hydrodesulfurization at atmospheric pressure was chosen as a test reaction. The K+-free catalysts, which contained a surface polymolybdate structure in the oxidic precursor, showed the highest HDS activity. The presence of potassium tended to produce monomolybdate structures with Mo6+ in tetrahedral oxygen coordination. These structural modifications had a strong negative effect on the reducibility of the catalysts and on their activity for thiophene hydrodesulfurization. The presence of potassium had a much larger negative influence on the sulfidability than on the reducibility of the molybdenum compounds.