The functional properties of metal oxide semiconductors depend on intrinsic and extrinsic defects. The population of intrin-sic defects is strongly affected by the synthesis method and subsequent treatments of the material, while extrinsic defects can originate from suitable doping. Stoichiometric ZrO2 is a non-reducible oxide with a large band gap. Therefore, controlling and modulating its defect profile to induce energy states in the band gap is the sole possibility to make it a photocatalyst responsive to visible light. We report a method, based on low temperature sol-gel synthesis coupled with treatments per-formed in mild conditions, to obtain undoped visible light responsive ZrO2-x. The electronic structure of these materials is interpreted in relation to their oxygen vacancy defect population. On the basis of a wide set of experimental measurements (X-ray photoelectron, steady state and time-resolved …