The passive oxide films formed by electrochemical oxidation on CoCrMo alloy (ASTM F75) in simulated physiological solution were studied using X-ray photoelectron spectroscopy (XPS). The oxidation was performed in a closed system without exposure to laboratory air. The fitting procedure used allowed the separation of the measured spectra, Co 2p, Cr 2p, Mo 3d and O 1s, into contributions of various species. Quantitative evaluation included the determination of the composition, cationic fraction and thickness of the oxide layer formed. An insight into the structure of the layer was offered by angle-resolved XPS measurements. The addition of complexing agent, in the form of Na-citrate, to the physiological solution affects the passivation of CoCrMo alloy. (C) 2003 Elsevier Science Ltd. All rights reserved.