Al₂O₃-supported molybdate catalysts are structurally characterized by physical methods, in particular Raman, infrared, photoelectron spectroscopy and perturbed angular correlation. In the oxidized catalyst precursor, the molybdate species are present as surface polymolybdate. Addition of K₂CO₃ leads to decondensation of the polymolybdates and stabilization of the monomeric MoO₄² anion. This structural change has significant consequences for the reduction and sulfidation behavior of the catalysts as well as for their catalytic properties. In particular, the K-promoted catalysts exhibit increased activity for the water-gas reaction, with surface formats presumed to be intermediate steps in the reaction. Sulfidation of K-free catalysts results in MoS₂-like structures that are highly disordered and presumably retain significant amounts of hydrogen. The defects in the MoS₂ structure (anion defects) are presumably to be regarded as coordination centres in chemisorption processes and possibly as active centres in catalytic reactions.