Solubility measurements are reported for four hydrated cement phases. A technique employing repeated dispersion and filtration is described which enabled the solubility (activity) products of ettringite (C6AS3H32) and hydrogarnet (C3AH6) to be calculated with confidence. Free energies of formation are also given. These phases dissolve congruently, whereas monosulphate (C4ASH2) and tetra-calcium aluminate hydrate (C4AH13) exhibit incongruent dissolution, and can therefore not be described by a conventional solubility product. The results represent additional data for inclusion in thermodynamic databases currently being applied to the modelling of cement systems.