A photoelectrochemical investigation on anodic films of different thickness grown on sputter-deposited Zr and Zr-20at.%Ti was carried out. The estimated optical band gap and flat band potential of thick (U-F >= 50 V) anodic films were related to their crystalline structure and compared with those obtained for thinner (U-F <= 8 V/SCE) anodic oxides having undetermined crystalline structure. The E,, values obtained by photocurrent spectroscopy were also compared with the experimental band gap estimated by other optical ex situ techniques and with the available theoretical estimates of the zirconia electronic structures in an attempt to reconcile the wide range of band gap data reported in the literature. Finally, the optical band gap and flat band potential values obtained for tetragonal Zr0.8Ti0.2O2 mixed oxide were compared with that expected on the basis of a correlation between E-g and the film composition. (c) 2007 Elsevier Ltd. All rights reserved.