[n-C16H33 N(CH3)(3)](3) PW12O40 (1a) catalyzed the oxidation of secondary alcohols with 27.5% aqueous hydrogen peroxide under solvent-free conditions. The isolated yields of all ketones were higher than 92%. The turnover number of the catalyst la was above 368, and the highest TON and TOF were up to 3840 and 320 h(-1). In this catalytic system, the catalytic active species was {PO4[WO(O-2)(2)](4)}(3-), which was formed from la in the reaction. It was discovered that {PO4[WO(O-2)(2)](4)}(3-) (PW4) and [PW12O40](3-) (PW12) kept an equilibrium during the alcohol oxidation by simultaneous monitoring. the distribution of species in organic and aqueous phases. The analysis of the W content in the aqueous phase by ICP and the detection of the species transformation in the organic phase by P-31 NMR revealed that the most of the PW4 species were transformed to the PW12 species again after the reaction. PW12 and PW4 were in the transform-and-retransform process. (C) 2008 Elsevier B.V. All rights reserved.