Transesterification of soybean oil by using the synergistic microwave-ultrasonic irradiation.

•Transesterification of soybean oil was performed with combined microwave-ultrasound.•MW power, US power and mode along with all reagents’ amount were investigated.•The highest transesterification yield of 98.0% was obtained.•The optimal MW and US power were 700W and 800W, respectively.•Microwave-induced activation was proved predominant to reaction acceleration.