The mono-, di-, tri- and tetraperoxomolybdate intermediates formed in the reaction between molybdate ions and hydrogen peroxide are identified by Mo-95 NMR. The influences of pH and hydrogen peroxide concentration are discussed in terms of equilibrium constants. Comparison of these results with kinetic studies, performed under the same conditions, leads us to conclude that the oxotriperoxomolybdate MoO(O-2)(3)(2-) is the main precursor of molecular oxygen. The decomposition rates of the different intermediates are also determined, and a mechanistic scheme is proposed.