The photocatalytic oxidation of cyclohexene and cyclooctene by (nBu(4)N)(4)W10O32 supported on silica is strongly affected by the presence of CH2Cl2 as co-substrate. CH2Cl2 undergoes oxidative pathway yielding radical intermediates. This species are involved in the subsequent epoxidation of cyclohexene and cyclooctene, which occurs with a selectivity higher than 50%. A major competing reaction is the well known oxidation of the alkenes to the corresponding allylic hydroperoxides. Cyclooctene epoxide is stable enough to be accumulated in the irradiated solution. On the contrary, cyclohexene epoxide undergoes ring opening to form 2-chlorocyclohexanol. The good stability of the heterogenized decatungstate is demonstrated by the fact that it could be employed at least three times without suffering any appreciable loss of photocatalytic activity. Decatungstate loading and initial cyclohexene concentration influence the chemoselectivity of the photocatalytic process. In particular a high number of active centers and a low cyclohexene concentration favor a chlorination pathway of cyclohexene, which leads to the formation of 20% of 3-chlorocyclohexene. (C) 2003 Elsevier B.V. All rights reserved.