Concentrated (0.2 M) aqueous solutions of HP-acids, such as (H3+x+mPVmVx-mMo12-xO40)-V-IV-Mo-V and their analogues with an excess VO2+ cation, are oxidized by dioxygen at 343 K and atmospheric pressure through intermediate active complexes (IAC) [(Hx+m-1PVmVx-mMo12-xO404-)-V-IV-Mo-V] . [VO2+](y) . O-2, where m + y greater than or equal to 3. The electron transfer to the coordinated O-2 molecule inside AC is the limiting stage at high m. At low m, the formation of IAC becomes the limiting stage that results in a sharp decrease in the oxidation rate.