Passive film formation on a chromium surface was studied by three-parameter ellipsometry as the film was formed by anodic polarization in citrate buffer solutions. The thickness and the optical constants of the film were obtained from automatic measurements as functions of time of anodic polarization, from subsecond to 60 s range. The thickness of the passive film increased rapidly within a few seconds, during which both the real and imaginary parts of the complex refractive index of the film changed. These film properties approached near stationary-state values after about 20 s. The stationary-state film thickness increased linearly with the potential of the chromium electrode and depended on pH of the electrolyte, whereas the optical constants did not show significant dependence on these factors. The passive film appeared to be a homogeneous single near-dielectric film, possibly of a composition close to Cr2O3. No indication of a transformation of the film was observed during the early stages of passivation.