Selective catalytic oxidation of various organic substrates with O-2 in the presence of aqueous solutions of Mo-V-P heteropoly acids (HPA) is carried out via two stages in separate reactors. In stage (1), a substrate is oxidized into a desired product while HPA is reduced. The reduced form of HPA is oxidized with O-2 in stage (2). A set of the physicochemical properties of the homogeneous catalyst has been found to alter continuously during these redox processes. Using a solution of the modified high-vanadium HPA (H12P3Mo18V7O85), we demonstrate that the density, viscosity, and pH of this solution reach their maxima after reaction (1) and attain their minima after reaction (2). On the contrary, the redox potential of the solution is minimum after reaction (1), and maximum after reaction (2). All alterations of the physicochemical properties of the catalyst are found to be completely reversible.