This work reports the hydrolysis of vegetable oils catalyzed by SO3H-functional Bronsted acidic ionic liquids at moderate hydrolysis temperature and reaction time. The results demonstrated that the catalytic activity of ionic liquids is in the sequence 1-(4-sulfonic group) butylcaprolactamium hydrogen sulfate [HSO3-bCPL][HSO4]) > 1-(4-sulfonic group) butylpyridinium hydrogen sulfate ([HSO3-bPy][HSO4]) > 1-(4-sulfonic group) butyl-3-methylimidazolium hydrogen sulfate [HSO3-bMIM][HSO4]) > 1-(4-sulfonic group) butyltriethylaminium hydrogen sulfate ([HSO3-bTEA][HSO4]). In the batch reactor, the reaction conditions of the hydrolysis in the presence of [HSO3-bCPL][HSO4] were investigated and optimized. The yield of fatty acids was greater than 95 wt % at [HSO3-bCPL][HSO4] dosage of 8 wt 96 (based on the weight of oil), water/oil molar ratio of 10:1, hydrolysis temperature of 180 degrees C, and hydrolysis time of 6 h. The catalytic performance of this ionic liquid remained excellent even after reuse six times.