The chemical stability of titanium is due to the spontaneous formation of a thin oxide film on its surface. The growth of anodic oxide films on titanium can be accompanied by the incorporation of species from the electrolyte into the oxide, affecting the properties of the final oxide. In this work, anodic titanium oxide films obtained at galvanostatic conditions (1.52 mA cm(-2)) in phosphate buffer solutions (pH 1 and 5) were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS data yielded that the anodic oxide films had phosphorus incorporated into them and were composed mainly by TiO2. Furthermore, when the oxide film surface was eroded by Ar+ sputtering, the phosphorus content decreased with depth. (C) 2004 Elsevier B.V. All rights reserved.