The aim of the present work was to provide a better understanding of the biocompatibility of Co-based alloys in the physiological environment. Co-based alloys are widely used in orthopaedics for the manufacture of prosthetic devices. Two alloys, Co-Cr-Mo and Co-Ni-Cr-Mo, and their individual metal components were studied in simulated physiological solution at 37 degreesC, with and without the addition of two complexing agents, EDTA and citrate, in the concentration range from 0.1 to 50 mM. The processes were studied using electrochemical techniques: cyclic voltammetry, anodic potentiodynamic measurements, linear polarization and electrochemical impedance spectroscopy. The results are discussed in the context of the dependence of the passivation behaviour of the two alloys on their composition. Both alloys are shown to passivate very well in simulated physiological solution. The effect of complexing agents is more evident on Co - Ni - Cr - Mo than on Co - Cr - Mo alloy. This appears to be related to the presence of Ni.