The influence of gypsum content on the hydration and compressive strengths was studied on two commercial Portland cements having the same amount of alite, a similar amount of C 3A, but different amounts of readily soluble alkalies and sulfates. The phase analysis of samples hydrated during the 90-days' period was carried out by X-ray diffraction, selective extraction and simultaneous DTA-TG-DTG method. It was observed that all the effects noticed on pure C 3S- S -H system are determinant for the optimum gypsum content, although for the sample containing a higher amount of readily soluble alkalies the influence of the added SO 3 is considerably modified by the delayed ettringite formation caused by the interaction between Al, Fe-bearing phases and S -rich C-(S, S )-H.