The tetrahexylammonium (THA) salt of the peroxocomplex [W2O3(O-2)(4)](2-) was used as catalyst for the oxidation of cis-cyclooctene, geraniol and cyclooctane, with hydrogen peroxide, in acetonitrile. High catalytic activity for all substrates, with high efficiency of use of hydrogen peroxide, was found. Under the conditions used, the oxidation of cyclooctane gives rise to cyclooctanone, cyclooctanol and cyclooctyl hydroperoxide, with 90% and 97% of conversion after 6 and 12 h, respectively, reaching turnover numbers higher than 600. A possible mechanism for the oxidation of cyclooctane is proposed. The oxidation of cis-cyclooctene gives the corresponding epoxide with 100% conversion after 4 h, whereas geraniol yields mainly the 2,3-epoxide (94% selectivity) with 88% of conversion after 5 h of reaction. The crystal structure of (THA)(2)[W2O3(O-2)(4)] is reported. The anion has two types of coordinated peroxide ligands, namely symmetrical and unsymmetrical bridging peroxo groups. This study indicates that the peroxocomplex [W2O3(O-2)(4)](2-) is catalytically active both in epoxidation reactions in acetonitrile and in the oxyfunctionalization of a cycloalkane. (C) 2008 Elsevier B.V. All rights reserved.