The tetrahedral triphenylsiloxy complex MoO2(OSiPh3)2 (1) and its Lewis base adduct with 2,2′-bipyridine, MoO2(OSiPh3)2(bpy) (2), were prepared and characterised by IR/Raman spectroscopy, and thermogravimetric analysis. Both compounds catalyse the epoxidation of cis-cyclooctene at 55 ºC 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·molMo‑1·h‑1 for a catalyst:substrate: oxidant molar ratio of 1:100:150) is much more active than 2 (initial activity 12 mol·molMo–1·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·mol–1) as compared with 2 (20 kcal·mol–1) is in accordance with the higher activity of the former.