Bulgarian lignites (Maritza East, Maritza West, Sofia) were gradually heated under air from 100 degrees C to their fluid ash fusion temperatures (1200-1300 degrees C) via 100 degrees C intervals and the behaviour of their inorganic matter (IM) was studied. The pre-existing minerals and newly formed inorganic phases in the oxidation and combustion products (OCPs) of these coals were identified and the behaviour of 38 minerals and phases was described. The lignites reveal low detrital abundance and high authigenic mineralization with sulphide-sulphate or carbonate tendencies. The IM of lignites is composed mainly of quartz, kaolinite, gypsum, calcite, and pyrite, while the other minerals identified have subordinate occurrence. The IM of OCPs includes also various newly formed phases such as glass, amorphous clay material, mullite, hematite, Ca and Ca-Mg silicates, cristobalite, tridymite, magenetite, Ca and Mg oxides-hydroxides, anhydrite, and others originating from the heating of these lignites or storage of their OCPs. The physico-chemical processes and temperatures that result in the fort-nation of new phases in OCPs are described. The relationships between the ash fusion behaviour and chemical and mineral composition of the lignites are also discussed. (c) 2005 Elsevier B.V. All rights reserved.