The equilibrium compositions of the products of combustion of brown coals from the Rhineland (West Germany) have been calculated over a wide range of temperatures and stoichiometrics. Special attention has been given to the fate of the mineral matter in the coals and, in particular, to species containing the elements Na, K, S and Ca. The results show that the species of importance at equilibrium are very much a function of the initial composition of the coal but that the important species can be categorized broadly as a function of temperature and stoichiometry. For the condensed phases, sulphates predominate at all temperatures under fuel-lean conditions. For fuel-rich conditions carbonates, sulphides, and oxides and hydroxides are of chief importance in the low (<110O K), middle (1000-1200 K) and high (< 100 K) temperature ranges respectively. Silicates and silica are the most persistent condensed species, occurring across a very wide range of temperatures and stoichiometrics. The calculations are consistent with boiler fouling mechanisms that have been proposed to explain observations made with the Rheinisch-Westfälisches Elektrizitätswerk experimental combustion rig.