The influence of thermal aging, at intermediate temperature (1 h at 250 degrees C) and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron-sputtered niobium metal, has been studied. A detailed physicochemical characterization of the a-Nb2O5/0.5 M H2SO4 electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well as by differential admittance (DA) measurements. A change in the optical bandgap (3.45 eV) of niobia film has been observed after aging (3.30 eV) at 250 degrees C in air for 1 h. A cathodic shift (0.15-0.2 V) in the flatband potential of the junction has been observed. The frequency dependence of DA data agrees with expectations of the theory of amorphous semiconductor Schottky barrier. The fitting of both components of DA allowed to get information on the distribution of the electronic density of states as a function of energy and distance from the metal oxide interface. The DA measurements evidenced for vacuum-treated niobia film an insulating to semiconductor transition. These findings can help to explain the large changes in the measured values of capacitance, after aging, and the larger leakage current observed in niobia electrolytic capacitors. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428396] All rights reserved.