Coal slurry is a fine-grained (<1 mm) by-product of coal washing. In the past, coal slurries were deposited in settling ponds as a waste. Currently, with the introduction of fluidized bed boilers in the Polish energy sector, coal slurries are used as a fuel. Compared to coal, coal slurries (analyzed in the paper) are characterized by a high content of macerals of the inertinite group, with inertodetrinite dominant. Among macerals of the vitrinite and liptinite groups, a high content of detrial macerals also merit attention. The mineral composition of coal slurries is close to that of coal, although coals are characterized by a slightly higher mineral variability. Unburned organic matter in the ash resulting from burning a mixture of coal and coal slurry is dominated by detritus (<10 mu m). The high content of detritus in the ash may partially result from a very high content of detrial macerals in coal slurries. A small share of solid forms and inerdoids in the ash reflect the generally reactive nature of the inertinite present in coal slurries. X-ray diffraction and scanning electron microscopy of ash samples reveal quartz, anhydrite, mica, hematite, feldspar, calcite and portlandite in the ash. Metakaolinite, hydrophilite, dolomite, iron oxides, calcium oxide and aluminosilicates occur in smaller quantities. (C) 2015 Elsevier B.V. All rights reserved.