The surface oxide film on titanium and its long-term stability in biological environments play a decisive role for the biocompatibility of titanium implants. In this study, the passive oxide film formed on titanium and its natural growth in a phosphate buffered solution with and without an H2O2 addition have been investigated by electrochemical impedance spectroscopy (EIS) measurements over a period of several weeks. In the absence of H2O2, the impedance response indicated a stable thin oxide film on titanium. However, the introduction of H2O2 into the solution resulted in significant changes in the EIS-spectra, which varied with exposure time. The interpretation of results is based upon a two-layer model of the oxide film, consisting of a thin barrier-type inner layer and a porous outer layer. The H2O2 addition in the solution led to a significant decrease in corrosion resistance of titanium and also to a thickening of the porous outer layer. The observations may provide an explanation of the unexpected in vivo titanium oxide growth and ion incorporation into titanium implant oxide surfaces.