Slags are formed during coal combustion processes as a result of the melting and reaction of the mineral matter. Predicting the outcome of these complex chemical reactions has long been a problem. Improvements in chemical thermodynamic models of oxide systems and computational methods now make these predictions possible. In this paper a model of the system Al2O3-SiO2-CaO-FeO-Fe2O3, developed using the F*A*C*T computer system, is presented. Examples are given of the use of the model to predict the phases formed as a function of ash composition, temperature and oxygen partial pressure of the system. These examples show how results can be presented in forms which can be readily used by practising combustion engineers, and coal production and marketing personnel.