Mechanism of the (Formula presented.)(~300H2O)·(~10CH3 COONH4) polyoxomolybdate degradation in an aqueous solution is proposed and substantiated for the time using kinetic modeling. The mechanism includes four stages: I) non-catalytic cleavage of the Mo–O bond between the polyoxomolybdate fragments; II) recovery of the bonds between the polyoxomolybdate fragments; III) catalytic cleavage of the Mo–O bond between the polyoxomolybdate fragments; IV) increase of O2 concentration due to the decay of the conjugate pair (Formula presented.) releasing the oxygen molecule and due to O2 diffusion from the gas phase into the polyoxomolybdate aqueous solution. A kinetic model of the considered process is proposed. It is shown that bond recovery prevails over bond cleavage in the polyoxomolybdate due to low residual O2 concentration in the solution. At the same time, increasing the O2 concentration in the solution up to some “critical” value restarts the avalanche-like autocatalytic process of Mo–O bond cleavage in the polyoxomolybdate.