When it comes to the production of energy from renewable sources, biomass is the main fuel, burned directly or co-fired with coal, used in the professional power industry. As in the case of coal, the use of biomass in the professional power industry is accompanied by the generation of by-products of the combustion process, primarily in the form of fly ash. These wastes significantly differ from those resulting from coal combustion. Their properties depend primarily on the burned biomass and boiler type. Due to the growing pressure on the use of energy from renewable sources resulting from the Energy Policy of Poland and the requirements imposed by the EU, more and more by-products are produced. Ashes from the co-firing of biomass are relatively well studied, especially when it comes to those resulting from the combustion in conventional boilers. The by-products of biomass combustion are of limited economic use due to their specific characteristics. The ashes resulting from the combustion in fluidized bed boilers are particularly problematic. The paper presents the research results on the basic properties of the three ashes generated from the combustion of biomass in fluidized bed boilers and one ash resulting from the co-firing of biomass with coal in pulverized coal boiler for the same biofuel type. The conducted analysis of the chemical composition has shown a high content of CaO and CaOw, SO3, and K2O and a low content of SiO2 and Al2O3 compared to ash from co-combustion of biomass. The elemental analysis indicates a high content of: P, S, Cl, K, and Ca and lower content of chromium and cobalt in the ashes generated from burning of biomass when compared with the ashes produced as a result of co-combustion. All the tested ashes have similar granulometric composition. Particular attention was paid to the leachability of pollutants, which is one of the most important factors determining the use of waste in mining technologies, using mainly the mixtures of fly ash and solid waste from calcium-based flue gas desulphurization (10 01 82). The pH of leachates from the analyzed ashes is the lowest for the ashes resulting from the co-combustion of biomass. The pH value of leachates was approximately 12 for all of the tested samples. The results have shown a high leachability of potassium and chlorides, which is characteristic for by-products resulting from the combustion of biomass, and a high leachability of sulphates due to the type of used boilers. The phase composition is dominated by calcium and potassium carbonates, quartz, K2SO4, halite, sylvite, CaO, MgO.