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Using Mott-Schottky (M-S) analysis in conjunction with the Point Defect Model (PDM) for passive films, we have determined the oxygen vacancy diffusion coefficient in the WO_3−x passive film formed on tungsten in phosphoric acid (pH = 0.96) solution at ambient temperature. The value obtained for D_o is in the range (3.7–5.3)× 10⁻¹⁵cm²s.This value was extracted from the exponential relationship between the donor density and the film formation voltage assuming that the depletion region in the film close to the metal/film interface dominates the semiconductor properties.