Triple-quantum two-dimensional Al-27 magic angle spinning nuclear magnetic resonance (Al-27 3Q-MAS NMR) was used to characterize the substitution of Si4+ by Al3+ into the Te-Oc-Te structure of calcium silicate hydrates (C-S-H). This substitution was studied with C-S-H having an Oc/Te ratio of I and in equilibrium with Al(OH)(3) in aqueous suspensions. In the absence of NaOH, no substitution into the C-S-H structure occurred. Addition of NaOH in the preparation increased the concentration of Al(OH)(4)(-) and favored substitution. The deficit of charge resulting from this substitution was compensated by the accommodation of sodium in the interlayer space of the C-S-H. Increasing levels of substituted silicon correspond to higher alkaline and lower calcium contents in the interlayer space. Two substitution sites were distinguished, corresponding to the bridging and nonbridging positions in the chains of tetrahedra. A high Al-tetra(Si+Al-tetra) ratio indicated a redistribution of the aluminum tetrahedral sites to stabilize the substituted structure favoring aluminum in the bridging position.