The molybdenum/lanthanum-based (Mo/La) composite conversion coating on AZ31 magnesium alloy was investigated and the corrosion resistance was evaluated as well. The morphology, composition and corrosion resistance of the coating were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization analysis, respectively. The results revealed that the conversion coating consisted of spherical nodular particles, which was mainly composed of Mo, La, O and Mg. After conversion treatment the corrosion potential shifts about 500mV positively, and the corrosion current density decreases two orders of magnitude. The corrosion resistance of AZ31 alloy is remarkably improved by Mo/La composite conversion coating. (C) 2010 Elsevier B. V. All rights reserved.