29Si NMR has been applied to the study of calcium silicate hydrate (C-S-H) gels. The degree of silicate polymerization in young gels is found to be sensitive to the curing humidity; higher degrees of polymerization are observed for gels cured in drier environments. Compositional effects on polymerization have been investigated and found to be insignificant at CaO/SiO2 ratios greater than about 1.2. At lower lime contents, the predominant silicate species are long chain polymers. The results of the studies on synthetic C-S-H are discussed in relation to the hydration of Ca3SiO5 and historical cement specimens.