Abstract: An improved method and apparatus for effecting a substantial reduction in retention time of oxidation processes by the autoclave oxidation of ion species dissolved in aqueous solutions. In one embodiment, the invention provides for oxidation of dissolved reduced ion species including ferrous iron (Fe.sup.+2), uranium (U.sup.+4) and vanadium (V.sup.+3) from wet process phosphoric acids, or for conditioning of aqueous solutions for other uses, such as corrosion control, by raising the valence state of dissolved ion species. In accordance with the present invention, pure oxygen, present in quantity of at least 94% by volume, is utilized as an oxidizing agent with specified process parameters: temperature, pressure and mixing conditions in an autoclave reaction vessel.