The Wacker oxidation of cyclohexene to cyclohexanone, using the chloride ion-free catalytic system Pd(NO3)(2)/CuSO4/H3PMo12O40, was investigated at different air pressures, temperatures, and catalyst concentrations. The results show that this system is very efficient and highly selective. After 1 h of reaction at 80 degreesC and an air pressure of 50 bar, a conversion of 80%, with a turnover frequency of 260 h(-1), and a selectivity of more than 99% for cyclohexanone was obtained. Using aqueous hydrogen peroxide and no external pressure, the oxidation was more rapid, giving 80% conversion already after 30 min and 95% conversion after 60 min without the formation of any byproducts. (C) 2004 Elsevier B.V. All rights reserved.