The utilization of oil-cake waste (OCW) derived catalysts in biodiesel production was demonstrated. The sulfonated carbon materials were utilized as catalysts for pretreatment of acid oils (oils containing 8.17-43.73 wt% of free fatty acids). The catalysts could be used to convert free fatty acids (FFA) present in acid oils into corresponding methyl esters within 6-8 h at 80 degrees C, thus reducing the FFA content to desirable levels below 2 wt%. The esterification activity was found to be dependent on the initial FFA concentration found in the acid oil as well as methanol-to-oil molar ratio. High methanol-to-oil molar ratios and the presence of higher initial FFA concentrations resulted in enhanced esterification rates. Acid oil containing 43.7 wt% FFA was easily converted into mixtures containing up to 71% methyl esters (FAME). The reported catalyst was successfully recycled in five consecutive experiments and exhibited high thermal stability. Most importantly, the catalytic activity of the solid acid was found to outperform homogeneous case when equivalent amount of H2SO4 under similar conditions was applied. Therefore, the OCW derived sulfonated carbon catalyst reported herein could be used as a potential substitute for corrosive, concentrated H2SO4 currently employed for acid oil pretreatment. (C) 2013 Elsevier B.V. All rights reserved.