© 2022 Elsevier LtdBurning solid fuels in domestic boilers during the heating season increases the concentrations of dust containing carbonaceous particles that exist as organic carbon (OC) and elemental carbon (EC). For economic reasons, plastic wastes are also incinerated with traditional energy fuels. In this study, fossil fuels (bituminous coal and lignite) and biomass (softwood and hardwood) together with an addition of different types of plastics (HDPE, LDPE, PP, and PET) were combusted in domestic boilers. Carbonaceous matter in deposits (char and soot) from the combustion of fuels and fuels with added plastics has been examined by the thermo-optical method using an OCEC Lab analyser. The addition of plastics significantly changes the concentrations of individual carbon forms. The temperature range of the EC2 fraction (740 – 840 °C) includes the charring of plastics and OC3 (480 – 580 °C) and corresponds to the temperature range for polymer decomposition. Deposits from bituminous coal combustion contained EC2 = 10.28 mg/kg of combusted fuels, and OC3 = 1.25 mg/kg of combusted fuels. After the addition of plastics to bituminous coal, the concentrations of EC2 = 23.91 mg/kg, OC3 = 3.13 mg/kg of combusted household fuels increased. Similar changes in the concentrations of the EC2 and EC3 fractions were found after adding plastics to biomass and lignite. The method can be used for the preliminary identification of “banned fuels” in domestic boilers.