The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers: The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-metal alloys gives satisfactory agreement with the theoretical expectations. The experimental results evidence the role of the alloying metal in determining the crystalline or disordered structure of passive films grown on Al,d-metal alloys. (C) 2003 Elsevier Science Ltd. All rights reserved.