Thermal stressing experiments were performed in batch reactors at 250-425 degrees C for 3-63 min to evaluate the influence of thermal decomposition on biodiesel viscosity and cold flow properties. Dynamic viscosity was measured by a micro viscometer, and cold flow properties were characterized by differential scanning calorimetry (DSC). The crystallization onset temperature determined by DSC correlates with cold flow properties. Results showed that the cis-trans isomerization reactions had a minimal effect on both viscosity and cold flow properties of biodiesel, but polymerization and pyrolysis reactions had significant influence on both properties. Polymerization reactions resulted in increases in both viscosity and the crystallization onset temperature, while pyrolysis reactions showed the opposite effect. The current study suggests that polymerization reactions should be avoided or minimized during high-temperature non-catalytic homogeneous transesterification reactions for biodiesel production. (C) 2013 Elsevier Ltd. All rights reserved.