Production of biodiesel from rice bran and karanja oils was studied in a stirred tank reactor at different agitator speeds. The reaction system involves two immiscible phases: oil and methanol. The minimum critical speed and the emulsion properties for the reacting system were measured. Based on these values the sauter mean diameter, interfacial area and now regime were obtained. The minimum critical speed of agitator for rice bran was 700-750 rpm and for karanja was 550-650 rpm. The lower rpm for karanja oil is due to the presence of gums in the non-edible oils. The flow regime for the non-reacting system was found to be transitional and turbulent for reacting. The sauter mean diameter decreased to 7-8 times, from 2010 to 240 pm with increased agitator speed in the initial stages of the reaction and then was found to be constant with further increase. The specific interfacial area obtained from sauter mean diameter showed an increase accordingly. The economics of the biodiesel process depends on the conversions which also dictate the yield. The characteristics determined in the present work are useful in understanding the biodiesel process and help in the scale up of the reactor using different feed-stocks. (C) 2011 Elsevier B.V. All rights reserved.