Powdered samples of high-purity ZnO, TiO₂(rutile) and MgO, outgassed in vacuo for 16 h at 650, 750 and 850 K, respectively, promote rapid isotopic oxygen exchange in 1 : 1 mixtures of (¹⁸O₂+¹⁶O₂) admitted at room temperature. Heterophase exchange and homomolecular equilibration each contribute to the observed exchange but the level and reproducibility of such activity on each surface is strongly dependent upon the wavelength and flux of photons incident on the prereduced metal oxide, e.g. over TiO₂ room-light suffices to maintain constant activity which, however, decays in the dark. Restoration of activity to ZnO or MgO by photons of much longer wavelengths than fundamental or exciton absorptions of the metal oxide, allied to the persistence of activity after illumination, is interpreted in terms of a photo-assisted promotion of electrons out of deep traps and back into very shallow or quasi-free surface states which contribute to isotopic exchange.