In this paper, a study on the electrochemical impedance of anodic decomposition of it-GaAs in 0.5 M sulfuric acid, combined with steady-state polarization techniques, is presented. Both dark and daylight conditions have been investigated for comparison of their influence on polarization curves and impedance by hole injection in a potential range from open circuit to breakdown. An inductive loop appears if a high enough positive potential is applied for both conditions, while recombination resistance is only observed for the daylight case. Quantitative simulation of the impedance spectra suggests that the rate-determining step for corrosion/photocorrosion is the generation of mobile intermediates, e.g., AsGa+, which are then replaced by competition between a Faradaic process involving immobile intermediate formation and a parallel chemical step to form AsGa-OH. (c) 2005 Elsevier B.V. All rights reserved.