A novel thermogravimetric method for the determination of unburned carbon in industrial ashes from biomass combustion using the strongly endothermic Boudouard reaction is proposed. The method does not require acid treatment to remove carbonates; it may also be used for samples containing a higher amount of portlandite, calcite and other mineral components typical for biomass-combustion ashes and is utilizable in the wide range of the content of unburned carbon. The correctness of the proposed determination was verified by carbon mass balance in 10 samples of industrial ashes. The interpretation of the course of the TG curves obtained was corroborated by differential scanning calorimetry. (C) 2013 Elsevier B.V. All rights reserved.