Some CaO–SiO2–H2O gels of controlled composition have been synthesised by the hydrolysis of mixed alkoxide precursors and by direct reaction between colloidal SiO2 and Ca(OH)2. The gels were equilibrated with water at 25 °C after different ageing treatments, and the water compositions analysed. A mathematical model describing the free energy of formation of the gel as a function of its composition has been developed which accounts for the experimental data. The model assumes that both SiO2(amorphous) and Ca(OH)2 can form limited solid solutions with an intermediate compound and hence only a small number of parameters is required to characterise the entire system.