Biodiesel production from soybean oil transesterification under the presence of a heterogeneous catalyst tri-potassium phosphate in subcritical methanol has been studied. The results showed that tri-potassium phosphate exhibited high catalytic activity for the transesterification of soybean oil. The effects of reaction variables, including the reaction temperature, the molar ratio of methanol to oil, the catalyst amount, and the water and oleic acid concentrations in the soybean oil, on the yield of fatty acid methyl ester (FAME or biodiesel) were systematically investigated. Under the optimal conditions, e.g., the temperature of 220 degrees C and methanol to oil molar ratio of 24:1, 95.6% of the FAME yield was obtained in 30 min with only 1 wt% K3PO4. (C) 2011 Elsevier Ltd. All rights reserved.