We report the production of a biodiesel fuel by acid-catalyzed esterification of high free fatty acid (FFA) karanja oil (KO). Pretreated KO was converted to biodiesel by a process of alkaline catalyzed transesterification. Optimum acid-catalyzed esterification was achieved using 1% Sulfated Zirconia (SZ) as a solid acid catalyst with a methanol-to-oil ratio of 9: 1, temperature at 60 degrees C and reaction time of 2 h. During this process, FFA were converted into fatty acid methyl esters. The acid value of karanja oil was reduced to 1.3 mg KOH/g from 12.27 mg KOH/g, which confirmed the conversion. Consequently, this pretreatment reduces the overall complexity of the process and reduces the cost of producing biodiesel fuel. (C) 2012 Elsevier Ltd. All rights reserved.