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Ash materials produced during combustion may create several technical and environmental problems. In this work, bottom and fly ashes obtained from lab-scale circulating fluidized bed oxy-fuel combustion of the mixture of sewage sludge and coal were characterized. The slagging and fouling tendency was determined; studies on the chemical composition, including heavy metals, were carried out; the environmental impact and their potential uses were also assessed. The findings showed that the lowest sintering temperature in the oxidizing atmosphere is mainly characterized by ashes from sewage sludge combustion: the higher the share of coal in the blend mixture, the higher the ash sintering temperature. A similar tendency occurs for the temperature of softening of the ash. The analysis of particle size distribution of fly ash showed that after the combustion of municipal sewage sludge alone, the volume fraction of <10 mm was 32%, while for fuel blends it was less than 10%. The shift of particle size distribution of ash from sludge combustion to fine particles results in significant activity of oxygen atoms and increased particle loading, as confirmed by the study of the amount of unburned char in ash. The fly ash after oxygen combustion was characterized by a low degree of crystallinity. The main crystalline phases were crystalline silica – SiO2, diiron trioxide – Fe2O3and calcium sulfate – CaSO4. The main components of fly ash and bottom ash were aluminum and silicon compounds.