The effect of addition of trace amount of KOH as co-catalyst on low boiling point organic amine-catalyzed transesterification for biodiesel production was investigated. Three different organic amines, tri-ethylamine, di-ethylamine and tert-butylamine, were used as catalysts, and the maximum amount of KOH that could be added to these organic amine-catalyzed systems was 367.1 mg/kg oil. Under such circumstance, KOH could be left in the resultant biodiesel and no washing was needed to remove it as the concentration of K(+) in the biodiesel met EN 14214 standards. Addition of trace amount of co-catalyst KOH with an amount of 367.1 mg/kg oil resulted in the organic amine-catalyzed transesterification under milder reaction conditions than those without addition of KOH to achieve > 90% yield of methyl ester. Furthermore, side reactions that occurred in the pure organic amine-catalyzed systems could not be detected after addition of trace amount of KOH, as evidenced by the NMR results. (C) 2010 Elsevier Ltd. All rights reserved.