CoCrMo alloy are one of the most important biomaterials for orthopaedic applications, widely used in total joint replacements. The thermal treatments applied to CoCrMo alloys modifies the microstructure of the alloy and change the electrochemical and mechanical properties of the biomaterial. Thus, the aim of this research is to characterize the electrochemical behaviour of CoCrMo alloy depending on the thermal treatment at different applied potentials (according to the cathodic, anodic and transpassive electrochemical domains of the alloy). Metallographic study and electrochemical techniques such as potentiodynamic curves, potentiostatic tests at different potentials (selected according to the electrochemical regions of the potentiodynamic curves) and electrochemical impedance spectroscopy (EIS) are employed to characterize the CoCrMo, alloy after three different thermal treatments in simulated body fluid (bovine serum solution). The same corrosion mechanisms were observed for the CoCrMo alloy under the three thermal treatments at different applied potentials, however, the rate of the corrosion reactions which take place on the metallic surfaces depends on those thermal treatments and it is related to the amount of carbide inclusions. At passive potentials the study reveals that the properties of the passive layer depend on the microstructure of the alloy. EIS results show that the resistance of the passive layer in bovine serum solution increased with the carbides solubilisation. (C) 2008 Elsevier Ltd. All rights reserved.