The kinetics of exchange of oxygen-18 between enriched oxygen gas and 38 inorganic oxides and 1 oxy-acid salt has been examined in detail. Most of the exchange reactions occur by a dissociative atomic mechanism confined to the surface layer of oxygen ions. For these there is a strong compensation effect between A0 and E in the rate expression: E shows a systematic fall with increasing size of the unit-crystal cell and related crystal parameters: the plots separate the oxides into structure-dependent groups. The slow stage is the desorption of oxygen. The oxides PbO, PdO, AgO, and CuO exchange the surface layer by a molecular reaction which also occurs to an appreciable extent, together with the atomic mechanism, on a number of other oxides. Na2WO4, V2O5, MoO3, and WO3 exchange the whole of the bulk oxygen with the gas phase by a combination of both mechanisms. SiO2 and GeO2 are inactive. Possible mechanisms for the two main reactions are discussed.