Electron paramagnetic resonance (EPR) spectroscopy under controlled atmosphere conditions has been applied to elucidate the nature and reactivity of oxygen species formed upon reoxidation of a reduced VOx(2.7 wt.%)/MCM-41 catalyst by gas-phase O-2 or N2O. An electrophilic Vn+center dot center dot center dot O center dot-(n = 4, 5) radical anion intermediate is formed from O-2 probably via dissociation of a bi-atomic adsorbed oxygen species. This radical is remarkably stable in inert atmosphere up to elevated temperatures but reacts partly with C3H8 and almost completely with C3H6 and CO already at room temperature. In contrast, no such species could be observed upon reoxidation of reduced VOx species with N2O. This might be due to the rapid formation of nucleophilic O2- oxide ions, which are not EPR-active. The different electronic nature of these oxygen intermediates is discussed as a reason for the higher propene selectivity obtained in propane oxidative dehydrogenation over VOx/MCM-41 with N2O. (C) 2010 Elsevier Inc. All rights reserved.