Synchrotron radiation-energy dispersive diffraction has been used for the first time to study the formation and stability of the calcium sulphoaluminate hydrates in hydrated Portland cement pastes. By using this technique it has also been possible to investigate microstructural and compositional characteristics of the ettringite (AFt) phase. The longer term slow development of the monosulphate (AFm) phase has also been monitored, although the characterized content is quite low. Differences were detected between the microstructural characteristics of the AFt phase formed in the high ferrite sulphate-resisting-type cement pastes, as compared with the equivalent phase formed from the ordinary Portland cements. These differences were especially significant at later hydration times and have been ascribed to compositional differences between the ettringite (AFt) formed from the two different types of cement.