The possibility of structure sensitivity in the heterolytic exchange of isotopic oxygen with the surface of a series of magnesium oxide samples has been probed. Two general approaches have been adopted: (i) variation in particle size for MgO samples exhibiting the 1100) termination plane and (ii) variation in termination plane by alteration of the preparation method. In the former case, the surface area normalised exchange rate is found to vary insignificantly for the samples tested, despite a variation in their surface areas of nearly an order of magnitude. In addition, at the temperatures employed, only a fraction of a monolayer of the available oxygen is found to exchange, and, once exchanged, sites can be re-exchanged back with very similar reaction kinetics. It is proposed that the mechanism of reaction may involve the formation and decomposition of intermediate ozonide species generated on reaction of gas-phase O-2 with surface O-, or is associated with the presence of ensembles of reactive low coordinate Mg2+O2- pairs. Studies involving samples prepared via different routes have demonstrated the order of activity to be I I I I I or [ 110) mean surface planes > [ 100) > I I I I), suggesting a beneficial role for ensembles of low coordinate Mg2+O2- ion pairs. In addition, it has been found that samples prepared from hydroxide-based precursors exhibit activity in the exchange process involving two lattice ions (R-2). This has been rationalised in terms of the exchange of diatomic surface species, such as peroxide ions and/or superoxide ions. (c) 2005 Elsevier Inc. All rights reserved.