Unburned carbon (UC) in six different combustion residues from solid biofuels has been examined using several methods of analysis, such as Loss On Ignition (LOI) and Thermogravimetric Analysis (TGA). Most of the samples came from the heating plants and local heat utilities in the Czech Republic. The amount of unburned carbon in the residues varied widely over an order of magnitude and in several samples accounted for about 7% of the ash mass. Experimental results have shown that LOI overestimated unburned carbon in nearly all samples. Some of these findings can be associated with previously identified thermal decomposition of minerals present in ash. Volatile organic compounds also appear to play a role in the final UC value. Out of TGA methods, first, we used strongly endothermic Boudouard reaction method as an advanced TGA method. However, this reaction was affected by the high potassium content in the samples which could cause incomplete conversion of carbon during Boudouard reaction, because of sintering and formation of the deposits of glass at high temperatures. The second method was a simple TGA method with a controlled atmosphere of nitrogen or air. For ashes, which tend to form a glassy coating and undergo a sintering process, it is not suitable to carry out the measurement at high temperatures, because the Boudouard reaction does not proceed entirely. For these types of samples, it is more appropriate to use TGA methods with a controlled atmosphere of nitrogen or air.