A combination of methods, including separation procedures, light microscopy, SEM, TEM, XRD and DTA-TGA methods, were used to characterize the phase-mineralogical and chemical composition, microstructural and some genetic phase peculiarities of solid waste products from coal burning. Fly ashes, bottom ashes and lagooned ashes from eleven Bulgarian thermoelectric power stations were studied. These products comprise inorganic and organic constituents. The inorganic part consists mainly of non-crystalline (amorphous) components and lesser amounts of crystalline components represented by various major, minor and accessory mineral phases. The organic constituent contains unbumt coal components represented by slightly changed, semicoked and coked coal particles. The origin of solid phases could be: primary - minerals and phases contained in coal and having undergone no phase transition (silicates, oxides, volcanic glass, coal particles); secondary - phases formed during burning (magnetite, hematite, metakaolinite, mullite, anhydrite, lime, periclase, Ca-Mg silicates, glass, semicoke, coke); or tertiary - minerals and phases formed during the transport and storage of fly ashes and bottom ashes (sulphates, carbonates and oxyhydroxides).