Highly selective epoxidation of allyl acetate was carried out over MoO3/MgO using tert-butyl hydroperoxide as an oxidizing agent. The catalytically active species was identified as monolayer MgMoO4 on MgO. MgO with large pores is suitable for selective epoxide formation. The highest yield of glycidyl acetate was obtained over 7 wt.% MoO3/MgO, in which MgMoO4 species were highly dispersed on the surface of MgO. Trialkyl borates were good promoters for selective epoxide formation. Among them, triisopropyl borate afforded the highest yield of 92%. H-1 and B-11 NMR measurements of a homogeneous MoO2(acac)(2)-B(OBun)(3)-(t-C4H9OOH) system showed that alkyl groups of the borate exchange with t-butyl hydroperoxyl group, and that the boron atom interacts with the oxygen of Mo=O, promoting reactivity of an oxygen atom in the hydroperoxyl group to attack the double bond of allyl acetate. (c) 2005 Elsevier B.V. All rights reserved.