Powdered zirconia and colloidal zirconia aqueous sols have been examined by diffuse reflectance and absorption spectroscopies and by photoluminnescence methods in solid/gas systems. The former system was examined following high-temperature treatment in vacuo and under reducing and oxidizing atmospheres. Studies of the influence of H₂ and O₂ on the photophysics of microparticles (powder) and nanoparticles (colloidal sols) of zirconia at solid/gas interfaces and the effect of free carrier scavengers (CH₃OH and O²) on the photophysic at solid/liquid interfaces were undertaken to explore the correlation between surface chemistry and the nature of preexisting or photogenerated defect centers (e.g., F-type and V-type color centers). ZrO₂ is an insulating, direct wide gap metal oxide with an optical band gap of ∼5.0 eV; another optical transition occurs at 5.85 eV. The optical behavior depends on whether zirconia is preirradiated in the intrinsic (hv > 5.0 eV) or extrinsic (hv < 5.0 eV) absorption regions. The red limits of the effect are 3.0 and 3.2eV for microparticles and nanoparticles respecttively. New defects are formed by the photoionization of, and/or by free carrier trapping by, existing defects. New defects formed by tunneling electron transfer from donor to acceptor defect states in zironia nanoparticles are not precluded. Regardless of the type of mechanism, the influence of surface chemical reactions on the formation of defect centers is typical of both systems which luminesce under irradiation. Powdered ZrO₂ shows a decrease in luminescence the longer it is irradiated Emission decay in ZrO₂ sols depends on whether the sols were preirradiated in the intrinsic or extrinsic regions; luminescence intensity was affected by the type of carrier scavengers present (methanol or oxygen). Different origins have been identified for the decay of emission: (i) for powdered ZrO₂ samples, nonradiative recombination of free electrons with photogenerated hole centers after preirradiation with UV light; (ii) for preirradiated colloidal ZrO₂ sels, photoionization of, and recombination of free carriers with, emissive defect centers.