In order to study the pore solution, the release and binding of alkalis in a hydrating cement system have been studied. First, the binding factors for sodium and potassium as determined by Taylor [Adv. Cem. Res. 1 (1987) 5_16] and the corresponding distribution ratios as determined by Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893_1903; Cem. Concr. Res. in press] are related to each other. It follows that the sorption of sodium is practically identical, whereas for potassium Taylor [Adv. Cem. Res. 1 (1987) 5_16] predicts a substantial lower degree of sorption. The concept of alkali release, pore solution decrease and sorption by formed C_S_H, is incorporated in the NIST hydration model (CEMHYD3D). Subsequently, the model is compared with OPC hydration experiments reported by Larbi et al. [Cem. Concr. Res. 20 (1990) 506_516]. Good agreement is obtained when the distribution ratios of Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893_1903] are applied. The results suggest that C_S_H is the only binder of alkalis in hydrating OPC.