The open circuit behavior of hafnium at different surface film thicknesses, immersed in 0.5N H2SO4 has been investigated using potential and impedance measurements. When the anodic current is interrupted, a decrease in oxide film thickness (thinning) was attributed mainly to chemical dissolution of the barrier film. A full inertness of the surface is attained when the polarization potential reaches around 25 V (NHE) where the surface film becomes insoluble in the surrounding electrolyte. It was found also that with gradual accumulation of the oxide layer, a pronounced increase of the open circuit potential was observed. At the thickness corresponding to the polarization potential 25 V (NHE), the rate of increase of open circuit potential after cessation of the current becomes very much reduced and thus a perfect, compact, and continuous oxide film was assumed to be formed on the metal surface.