The selectivity of a catalyst in ethylene epoxidation reaction was addressed using quantum mechanical computer simulations. We found that the catalyst's selectivity in the reaction of oxametallacycle to form ethylene epoxide (EO) rather than the competing acetaldehyde (Ac) is determined in part by the differential bonding affinity of the catalyst toward the O and C atoms of the oxametallacycle. This interplay between O- and C-metal bond strength is due to the different structures of the two transition states. Based on this finding, we introduce a new indicator that determines the difference between the EO and Ac activation energies in the oxametallacycle reaction remarkably well for a number of different materials. (C) 2008 Elsevier Inc. All rights reserved.