The capacitance of the passive film on tungsten in 1 M phosphoric acid solution has been measured as a function of frequency and voltage and Mott-Schottky analysis is used to derive dopant type and concentration data. The passive films were grown potentiodynamically to formation potentials within the range 3-10 V and were allowed to age until steady-state conditions were achieved. The capacitance was measured during a potential sweep from the formation voltage in the negative direction at a sufficiently high sweep rate that the thickness of the film remains constant. The data are interpreted in terms of a modified point defect model that takes into account the existence of an insulating outer layer on the defective WO3 barrier layer. The primary defect in the barrier layer was found to be the oxygen vacancy, which is concentrated in the region of the barrier layer adjacent to the metal, with the outer layer being postulated to form either by restructuring of the outer surface of the barrier layer or by hydrolysis of emerging cations or both. The data suggests that the space charge capacitance in the barrier layer is dominant at low formation voltages, but that at higher voltages the capacitance of the insulating outer layer makes an important contribution to the measured capacitance. (C) 2000 Elsevier Science Ltd. All rights reserved.