The catalytic homogeneous oxidation of cyclohexane and cyclooctane in acetonitrile with hydrogen peroxide in the presence of the tetrabutylammonium salts of Keggin-type polyoxotungstates [XW11O39](n-) and [XW11MO39]((n-m)-) (X = P, Si, B and M-III = Fe, Mn) afforded substantial amounts of the corresponding alkyl hydroperoxides. The cyclododecane oxidation reactions only gave rise to the alcohol and ketone. Under the used reaction conditions, high turnover numbers were obtained, reaching 614 in the cyclooctane oxidation after 9 h of reaction. These oxidation reactions appear to be radical processes, since they were totally inhibited in the presence of 2,6-di-tert-butyl-4-methylphenol or I-2, two well-known radical scavengers. (C) 2004 Elsevier B.V. All rights reserved.