Biodiesel has been produced by esterification of natural fatty acids with methanol in the presence of a modified bentonite with 1-benzyl-1H-benzimidazole-based Bronsted acidic ionic liquids as catalysts. The effect of reaction temperature, type and amount of catalyst, molar ratio and reaction time was investigated. The results showed that MB3B (bentonite modified with 3,3'-(butane-1,6-diyl) bis(6-sulfo1-(4-sulfobenzyl)-1H-benzimidazolium) hydrogensulfate) has the highest catalytic activity and best recyclability under the optimized reaction conditions. Thus, this modified bentonite is able to catalyze the esterification of oleic acid to its methyl ester in 6 h with yields of more than 92%. The catalytic activity of MB3B for the esterification of natural other fatty acids and alcohols has also been studied. (C) 2011 Elsevier Ltd. All rights reserved.