Phase equilibria in the CaO-Al2O3-Fe2O3-SO3 system have been studied, mainly at 1325 degrees C. In particular the solid solution compositions of ye'elimite Ca-4(Al6O12)SO4 and brownmillerite (C-2(A,F)) phases have been analysed and used to estimate the composition boundaries of these phases in their co-existence with liquid at 1325 degrees C. Sulfate and iron oxide facilitate the formation of a liquid phase, whose amount increases with overall Fe2O3 content. However, the solubility of sulfate in this liquid phase is very low and liquid formation tends to lead to the volatilisation of sulfate in unsealed samples. (C) 2013 Elsevier Ltd. All rights reserved.