The tetrahedral triphenylsiloxy complex MoO(2)(OSiPh(3))(2) (1) and its Lewis base adduct with 2,2'-bipyridine, MoO(2)(OSiPh(3))(2)(bpy) (2), were prepared and characterised by IR/Raman spectroscopy, and thermogravimetric analysis. Both compounds catalyse the epoxidation of cis-cyclooctene at 55 degrees C using tert-butylhydroperoxide (t-BuOOH) is decane as the oxidant, giving 1,2-epoxycyclooctane as the only product. The best results were obtained in the absence of a co-solvent (other than the decane) or in the presence of 1,2-dichloroethane, while much lower activities were obtained when hexane or acetonitrile were added. With no co-solvent, catalyst 1 (initial activity 272 mol x molMo(-1) x h(-1)for a catalyst:substrate: oxidant molar ratio of 1:100:150) is much more active than 2(initial activity 12 mol x molMo(-1) x h(-1)). The initial reaction rates showed first order dependence with respect to the initial concentration of olefin. With respect to the initial amount of oxidant, the rate order dependence for 1 (1.9) was higher than that for 2 (1.6).The dependence of the initial reaction rate on reaction temperature and initial amount of catalyst was also studied for both catalysts. The lower apparent activation energy of 1 (11 kcal x mol(-1)) as compared with 2 (20 kcal x mol(-1)) is in accordance with the higher activity of the former.