The effect of alcohol on transesterification of used vegetable oil under simultaneous microwave and ultrasound irradiations was studied. Ethanol or methanol was used as an alcohol. The synergistic effect of microwaves and ultrasound was evaluated and compared through a series of experimental studies. A molar ratio of 9:1 (alcohol to oil) was found to be optimum for both ethanol and methanol with approximately 98% and 96% biodiesel yields respectively. Similarly, a catalyst weight of 0.75% resulted in maximum biodiesel yields of 97% and 96% for methanol and ethanol respectively. A reaction time of 2 min was adequate for the transesterification reaction either using methanol or ethanol as a reactant. Higher reaction time severely affected the biodiesel yields when ethanol was used as a reactant. The synergistic effect studies showed that an equal rate of microwave and ultrasound exposure resulted in higher biodiesel yields for both reactants while ethanol produced superior results when microwaves or ultrasound irradiations were used as energy sources individually. There were more polyunsaturated fatty acids in the biodiesel composition for FAMEs compared to FAEEs. Power density tests optimized energy utilization by matching the energy output rates and the reaction volumes for transesterification reaction. (C) 2015 Elsevier B.V. All rights reserved.