[en] The degree of molybdenum oxidation in model nitrogen-fixing systems has been determined by X-ray photoelectron spectroscopy (r.p.s.) in the Ti(OH)-Mg(OH)-Mo system. The dependence of the Esub(bond)Mo3dsub(5/2) bond energy on the molybdenum formal oxidation degree is given. A change in bond energy by 0.8 eV corresponds to a change in molybdenum oxidation state by 1. The results of r.p.s. have been compared with kinetic data in order to determine the oxidation state of molybdenum ions. It has been determined that the active form of nitrogen-fixing hydroxides contains molybdenum in an oxidation state (3) whose content correlates with catalytic activity