The dimerization of fatty acid methyl ester was investigated using Bronsted-Lewis acidic ionic liquids (Is) as catalysts. It is found that IL (3-sulfonic acid)-propyl-triethylammonium chlorozincinate [HO3S-(CH2)(3)-NEt3]Cl-ZnCl2 (x = 0.64, x: molar fraction of ZnCl2, x = moles of ZnCl2/(moles of ZnCl2 + moles of ammonium salt) was of good catalytic performance. Under the optimum conditions m(biodiesel):m(IL) 15:1, biodiesel 15 g, reaction temperature 240 degrees C, and reaction time 5 h, the yield of product dimeric acid methyl ester was 96.8%. The reusability of IL was good and after it was used five times, the dimeric acid methyl ester yield was still more than 94%. Otherwise, a synergetic effect of Bronsted and Lewis acid sites enhanced the catalytic performance of the IL. (C) 2011 Elsevier B.V. All rights reserved.