Reduction of heteropolyacid H-3[PMo12O40]center dot aq has been followed quantitatively by P-31 NMR in its dilute aqueous solutions. The HPA anions ("blues") reduced by one, two and four electrons were detected, and their relative concentrations were evaluated at different total degrees of reduction. The rate constants of the intermolecular electron transfer have been estimated for differently reduced species from their NMR line widths and intensities and compared with the calculated rate constants for the outer-sphere electron-transfer mechanism. (C) 2013 Elsevier Ltd. All rights reserved.