The growth of the passive film on tungsten in phosphate buffer solutions (pH range from 1.5 to 5.1) has been described in terms of the High Field Model (HFM) and the Point Defect Model (PDM). The steady-state current and passive film thickness have been measured as a function of voltage, with the film thickness being obtained from an analysis of capacitance and reflectance data. The observed data cannot be accounted for by the HFM in its classical form, but can be understood in terms of the PDM. A modified HFM, taking into account film dissolution at the film/solution interface, has also been developed, but it, too, fails to fully account for the experimental observations.