The compatibility of phases within the CaO–SiO₂–Al₂O₃–SO₃–Fe₂O₃ system is revisited. The influence of the SO₂ partial pressure on the formation of phases and the stability of phase assemblages is addressed by means of new experimental methods (performed under controlled atmospheres) and thermodynamic calculations. Existing data from the literature are compiled and compared with the results obtained from this work. The results obtained are linked to real clinker formulations, and practical implications are discussed.