MoO2(acac)(2) has been encapsulated in silica matrixes prepared by hydrolysis/condensation of tetraethylorthosificate. Alternatively, hybrid organic-inorganic matrixes were prepared by adding 3-(triethoxysilyl)propylethylenediamine or 1,4-bis(triethoxysilyl)benzene to the reaction mixture. All systems were tested in the epoxidation of cyclohexene or cis-cyclooctene at 70 degreesC using tert-butyl hydroperoxide as the oxidant. In most experiments, 1,2-dichloroethane as a solvent and a molar ratio of [Mo]:[TBHP]:[olefin] = 1: 130:130 were employed. The catalysts immobilized in inorganic matrixes prepared in acidic conditions showed high activity in the first run, which could be attributed to the occurrence of homogeneous catalysis; basic conditions led to a significant decrease in activity with a concomitant decrease in the specific surface area of the matrix. Hybrid systems were less active in the first run, but their activities remained almost constant in at least four consecutive runs. Selectivities for epoxides were higher than 97% in all the cases. (C) 2002 Elsevier Science B.V. All rights reserved.