A novel eco-friendly method of the synthesis of aqueous solutions of the Keggin-type Mo-V-P heteropoly acids H3+xPVxMo12-xO40 (HPA-x) is proposed. At the first stage, V2O5 is dissolved in cooled H2O2 to form peroxyvanadic compounds, which spontaneously decompose to yield a H6V10O28 solution. The latter is stabilized by the addition of H3PO4 to yield a H9PV14O42 solution that is added to a boiling aqueous suspension of (H3PO4 + MoO3). This suspension is gradually evaporated producing the HPA-x solution. This safe and practically wasteless method holds much promise for the preparation of HPA-x solutions with x = 2-6.