Redox potentials E and pH values were measured in 0.2 and 0.01 M solutions of partly reduced heteropoly acids (H3+x+mPVmVx-mMo12-xO40)-V-IV-Mo-V (H(m)HPA-x, 1 less than or equal to x less than or equal to 4, 0 less than or equal to m less than or equal to x). The potentials were compared to the redox potential of a couple VO2+/VO2+ which is in equilibrium with various HPA-x(m) anions. It was found that all HPA-x(m) anions as VO2+ and VO2+ cations are one-electron oxidants/reductants.