The present study revealed the influence of different Al2O3 support on the basicity and CDG formation and the resultant effect on soybean biodiesel yield. It was found that n-Al2O3 facilitates the formation of CDG; whereas on a-Al2O3 and b-Al2O3 no CDG was detected. Among the three catalysts studied 20% CaO/n-Al2O3 showed maximum biodiesel yield of mass 90%. The contribution toward the greater biodiesel yield on CaO/nAl2O3 is mainly attributed to the CDG formation on its surface. The CDG formation on n-Al2O3 was more facile at 9:1 methanol/oil mole ratio and was clearly identified by 13C NMR spectroscopy.