In this paper, a new conversion coating-molybdate/phosphate (Mo/P) coating on magnesium alloy was prepared and investigated by electrochemical impedance spectra (EIS), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and salt-water immersion experiments, respectively. The results demonstrated that the Mo/P coating contained composite phases, which were consisted of metaphosphate as well as molybdate oxide with an "alveolate-crystallized'' structure. The composite Mo/P conversion coating had better corrosion resistance performance than molybdate (Mo) coating, and even had almost comparable corrosion protection for Mg alloy to the traditional chromate-based coating. (C) 2008 Published by Elsevier B. V.