The re-oxidation capacity of partially reduced heptamolybdo-pentavanado-phosphate polyanion (HPA-5(red) or heteropoly blue) by ozone in water and aqueous acetone solution has been examined under conditions simulating pulp bleaching process. Two oxidation stages/phases were revealed indicating the presence of two reactive states (forms) of catalyst in ozonation solution. The bulk of HPA-5(red) (ca. 90%) was completely recovered within a few first minutes of ozonation ([HPA](red) = 0.5 mM; 0.39 mmol O-3/min), whereas residual catalyst portion reacted very slowly (two orders lower rate) with ozone. Addition of organic solvent was shown to have a favorable effect on HPA-5(red) re-oxidation by ozone (k of 0.29 min(-1) vs. 0.43 min(-1) for ozonation, respectively, in water and 10% v/v acetone solution; pH 2), being evidently the principal reason for high efficiency of solvent-assisted HPA/O-3 pulp bleaching approach. The pH of reaction media (medium acidity) was found to have a notable effect on HPA-5(red) ozonation. (C) 2010 Elsevier Ltd. All rights reserved.