The catalytic synthesis of biodiesel from soybean oil by transesterification over basic ionic liquid catalysts had been studied at atmospheric pressure. Choline hydroxide (ChOH) catalyst exhibited better catalytic activity compared with other basic ionic liquid catalysts, and methanol is the best alcohol for biodiesel synthesis. The suitable molar ratio of methanol and soybean oil was 9:1, and the optimum catalyst dosage existed for catalytic activity, which was about 4 wt.% (without soap formation). The study also revealed that the appropriate reaction temperature was about 60 degrees C, and the suitable reaction time was 2.5 h on the basis of biodiesel yield. The reusability test showed that ChOH catalyst had perfect utility for repeated use. By basicity test, it was found that the basic ionic liquid ChOH possessed better basicity in methanol solution. The catalytic reaction mechanism was illuminated by the interaction between the methoxyl group after activating and the carbonyl group of the triglyceride, which has been investigated using quasi in situ infrared spectroscopy. (c) 2013 Elsevier Ltd. All rights reserved.