Powdered zirconia and colloidal zirconia aqueous sols have been examined by diffuse reflectance and absorption spectroscopies and by photoluminescence methods in solid/gas and solid/liquid systems. The former system was examined following high-temperature treatment in vacuo and under reducing and oxidizing atmospheres. Studies of the influence of Hz and Oz on the photophysics of microparticles (powder) and nanoparticles (colloidal sols) of zirconia at solid/gas interfaces and the effects of free carrier scavengers (CH3OH and O-2) On the photophysics 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 similar to 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 effects are 3.0 and 3.2 eV for microparticles and nanoparticles, respectively. 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 zirconia 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 sols, photoionization of, and recombination of free carriers with, emissive defect centers.