Redox reactions of Keggin-type polyoxometalates (POM), [PV _xMo _12-xO ₄₀] ^(3+x)- (x = 1, 2), have been studied for the first time employing an array of potentiometric chemical sensors. Variations in the concentrations of vanadyl ions (VO ²⁺) during the redox reactions of POMs were followed in the real-time. Differences in the vanadyl concentration profiles observed during re-oxidation of di- and mono-substituted POMs allow us to infer principally different re-oxidation mechanisms for these compounds. At the beginning of the oxidation of [PV ^IV ₂Mo ₁₀O ₄₀] ^7- under air an instant release of VO ²⁺ (ca. 7%) from the coordination sphere of POM to the outside solution was detected. VO ²⁺ concentration decreased rapidly while oxidation proceeded indicating return of vanadyl back to the coordination sphere of [PV ^V ₂Mo ₁₀O ₄₀] ^5-. This finding indicates the participation of VO ²⁺ outside Keggin structure in the re-oxidation mechanism of di-substituted POM. No release of VO ²⁺ was detected during re-oxidation of [PV ^IVMo ₁₁O ₄₀] ^5- by air oxygen in the presence of laccase (this POM is not oxidisable by air at low temperature and atmospheric pressure). Biocatalysis of [PV ^IVMo ₁₁O ₄₀] ^5- oxidation may include activation of the molecular oxygen by the prosthetic group of laccase thus allowing the oxidation of V(iv) inside the Keggin structure of POM, i.e., without vanadyl release into the outside solution.