Novel hybrid organic–inorganic calcium silicate hydrate (C–S–H) materials have been synthesized via a sol–gel process. The materials are obtained by precipitation in alkali media of a mixture of trialkoxysilane (ethyltriethoxysilane, n-butyltrimethoxysilane or 3-aminopropyltriethoxysilane) and tetraethoxysilane diluted in CaCl2 ethanol–water solution. XRD experiments show an increase of the basal distance of C–S–H with the content of trialkoxysilane suggesting the incorporation of organic moieties in the interlayer. 29Si NMR data show that the organic species do not disrupt the inorganic framework of C–S–H. 2D 1H–29Si HETCOR NMR experiments confirm that trialkoxysilanes such as ethyl- or aminopropyl-silane are incorporated in the silicate chains of the C–S–H structure. In the case of highly hydrophobic trialkoxysilanes such as n-butyltrimethoxysilane, the results suggest that a separation occurs between silicates and trialkoxysilanes, leading to a mixture of inorganic C–S–H on one hand and 100% organosilane calcium hybrid phase on the other hand.