In a cyclone system of combustion or gasification with molten-slag removal a large proportion of the coal is thrown to the slag-coated cyclone walls before it has time to burn. The welting properties of coal-ash slag on devolatilized coal, the evolution of gases from molten slag when in contact with coal particles, and the effect of mineral matter on combustion of coal have been investigated briefly using a heating microscope. Coal-ash slag does not wet coal or coal residues, thus combustion of the coal particles on the surface of molten slag is not retarded. Evolution of gases lakes place from the slag (a) on its initial melting with release of sulfur oxides, and above 1400 C where carbon monoxide is given off when the slag is in contact with coal. The latter is explained in terms of the formation of iron carbide at the slug/carbon interface, the carbide then diffuses into the slag where it reads with silica.