The growth of passive film on tungsten in phosphate buffer solution has been described in terms of the Point Defect Model (PDM). The steady-state current and passive film thickness have been measured as a function of voltage, with the him thickness being obtained from an analysis of capacitance and reflectance data. The observed data cannot be accounted for by the High Field Model (HFM) in its classical form, but can be understood in terms of the PDM. Diagnostic criteria that have been derived from the PDM were used to identify the majority charge carriers in the passive film. The Point Defect Model was employed, together with Mott-Schottky analysis to explore the crystallographic defect structures of the passive films, whereas their electronic structures were studied using photoelectrochemical impedance spectroscopy (PEIS). The experimental results demonstrate that these structures are strongly coupled with the vacancies acting as the dopants.