Thermal lens (TL) spectroscopy in a dual-beam configuration was applied in a study on biodiesel and oil-biodiesel blends. The goal of this work was to evaluate the behavior of the thermal and mass diffusivities that arose when oil was added to biodiesel and to verify the capability of the method to identify small concentrations of triacylglycerol in biofuel. When transitioning from pure biodiesel to a blend consisting of 98% biodiesel and 2% soybean oil, the thermal diffusivity decreased 15% and the mass diffusivity increased 59%, which indicated that both parameters can provide significant information about the presence of oil in biodiesel. Therefore, TL spectroscopy can be a useful method for certifying the quality of biodiesel and biodiesel blends. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.