This work describes the use of the residual mass observed in a thermal decomposition curve for a soy biodiesel sample as a measure of its degradation degree. The difference in thermal stability between esters and oxidation products is the key point to make the determination. Using residual mass, the acceptance or rejection of a biodiesel becomes a non-subjective process and does not require knowledge of the biodiesel oxidation process. The residual mass data were compared with those of kinematic viscosity, UV spectroscopy and nuclear magnetic resonance. The good agreement found between these data justifies the utilization of this method to determine the biodiesel oxidation degree. (C) 2015 Elsevier Ltd. All rights reserved.