Abstract The poorly crystalline calcium silicate hydrate (C-S-H) phases that form near room temperature, which include the technically important C-S-H gel phase formed during the hydration of Portland cement, have a broad similarity to the crystalline minerals tobermorite and jennite, but are characterized by extensive atomic imperfections and structural variations at the nanometer scale. Relationships between the aqueous solubility and chemical structure are reported for specimens formed by different preparation methods and with a broad range of compositions. Both new and previously published data show that these phases generate a family of solubility curves in the CaO–SiO2–H2O system at room temperature. As demonstrated by 29Si magic-angle spinning (MAS) NMR data and by charge balance calculations, the observed solubility differences arise from systematic variations in Ca/Si ratio, silicate structure, and Ca–OH content. Based on this evidence, the solubility curves are interpreted as representing a spectrum of metastable phases whose structures range from purely tobermorite-like to largely jennite-like. These findings give an improved understanding of the structure of these phases and reconcile some of the discrepancies in the literature regarding the structure of C-S-H at high Ca/Si ratios.