Solubility in the fully hydrated CaO-SiO2-H2O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)(2)(.)6SiO(2)(.)5H(2)O and 10Ca(OH)(2)(.)6SiO(2)(.)6H(2)O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients n(Si) and n(Ca). Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end-members are normalized to n(Si) = 1.0 +/- 0.3. Recommended stoichiometries and thermodynamic data for the C-S-H end-members provide a reliable basis for the subsequent multicomponent extension of the ideal C-S-H solid solution model by incorporation of end-members for the (radio)toxic elements or trace metals.