Methyl ricinoleate (MR) was synthesized from castor oil and methanol using ionic liquids as catalysts, by a transesterification reaction. The product was characterized using mass spectrometry. The efficiencies of four different catalysts, 1-methyl imidazole hydrogen sulfate salt ([Hmim]HSO4), 1-butyl-3-methylimidazolium hydroxide salt ([Bmim]OH), NaOH, and H2SO4 were compared. The effect of the methanol/castor oil mole ratio, reaction temperature, reaction time, and catalyst dosage on the MR content was investigated by single factor experiments. Based on single factor experiments and the Plackett-Burman design, the transesterification of castor oil and methanol was optimized using the response surface methodology. The results showed that the most effective catalyst was the ionic liquid [Hmim]HSO4. The optimal conditions were as follows: methanol/castor oil mole ratio 6:1, reaction time 4 h, reaction temperature 77 degrees C and [Hmim]HSO4 dosage 12%. Under these conditions, the MR content reached 89.82%. The catalytic activity of [Hmim]HSO4 still remained high after 4 cycles. (C) 2015 Elsevier B.V. All rights reserved.