Geochemical processes and mineralogical transformations in the spontaneously combusting coal waste pile of the Kukla mine in Oslavany, Czech Republic, were investigated. The aims of the study were: (1) the characterization of secondary minerals; and (2) determination of processes which influence the mobility of elements in the pile. The pile burned from the late 19th century until the 1990s and has acquired a zoned nature, with original black material in the core of the pile, red material produced by burning close to the pile slopes and grey and white efflorescent salts precipitated on the pile top and slopes. Several mineral assemblages have been identified including (1) primary minerals in the black material including bituminous coal to anthracite, micas, pyrite and goethite; (2) hematite, spinels and corundum in the red material produced by pyrometamorphism; and (3) hydrated sulphates of magnesium including hexahydrite, konyaite and picromerite in efflorescent precipitates on the slopes of the pile. A conceptual model of geochemical processes in the pile includes seasonal changes with mineral dissolution during the wet season and precipitation of efflorescent salt minerals during the dry season. Formation of secondary minerals such as hematite, which is resistant to weathering and immobilizes hydrolysable Fe, may have a positive environmental impact in the long term.