A new type of process for forming Ce-Mo conversion coatings on Al-alloys has been developed. Conversion coatings about 3.6 mum thickness were obtained by immersing Al-alloys for 20 minutes in boiling film forming solutions containing (NH4)(2)Ce(NO3)(6) 2.5g/l, NaKC(4)H(6)O(6)(.)4H(2)O 2.5g/l, Na2CO3 7.5g/l and Na2MoO4 5.0g/l. In the case of LF4 Al-alloy, polarization curves and immersion tests in 5% NaCl indicated that the conversion coatings exhibited more excellent resistance to localized corrosion than the conventional chromate conversion coatings. However, its resistance to localized corrosion was not satisfactory on LC4 Al alloy. Scanning Electron Microscopy (SEM) and Energy Dispersion Analyzer of X-Ray (EDAX) analysis revealed that the conversion coatings having complex surface microstructure on both LC4 and LF6 Al alloys consist mainly of O, Al and other alloying elements in addition to significant Ce and Mo. A mechanism of film formation was proposed to explain the experimental results.