Bulgarian subbituminous (Pernik, Bobov Dol) and bituminous (Balkan) coals were gradually heated under air from 100 degrees C to their fluid ash-fusion temperatures (1400-1600 degrees C) via 100 degrees C intervals and the behaviour of their inorganic matter (IM) was studied. The original minerals and newly formed inorganic phases in the oxidation and combustion products (OCPs) of these coals were identified and the behaviour of 33 minerals and phases was described. The coals studied reveal high detrital abundance and low authigenic mineralization with sulphide-sulphate, carbonate or mixed sulphide-sulphate and carbonate tendencies. The IM of coals is composed mainly of quartz, kaolinite, illite + muscovite, feldspars, pyrite, and calcite, while the other minerals identified have subordinate occurrence. The IM of OCPs includes various pre-existing minerals and newly formed phases. The latter phases are glass, quartz-cristobalite-tridymite, mullite, amorphous clay material, hematite-magnetite, anhydrite, and others originating from the heating of these coals or storage of their OCPs. The physico-chemical processes and temperatures that result in the formation of new phases in OCPs are described. The relationships between the ash-fusion behaviour and chemical and mineral composition of the coals are also discussed. A systematization of the physico-chemical transformations and some comparative characterizations, as well as prediction of certain technological and environmental problems related to the behaviour of IM during heating of Bulgarian lignites, subbituminous and bituminous coals are also described and summarized. (C) 2006 Elsevier B.V. All rights reserved.