Biodiesel could be synthesized using Bronsted acidic ionic liquid N-methyl-2-pyrrolidonium methyl sulfonate ([NMP][CH(3)SO(3)]) as a catalyst, specially with free long-chain fatty acids or their mixtures, as well as with low-molecular weight alcohols as substrates. This catalyst showed good catalytic and reusable performance under mild conditions and without any additional organic solvent. The ionic liquid could be reused eight times after the water in the ionic liquid was removed. The yields of fatty acid alkyl esters could reach between 93.6% and 95.3% after the esterifications were carried out at 70 degrees C for 8 h. Therefore, an efficient and environmentally friendly catalyst was provided for the synthesis of biodiesel from low-cost feedstocks such as waste oils. (C) 2009 Elsevier Ltd. All rights reserved.