The structures of aluminum‐substituted calcium silicate hydrate (C‐S‐H) forming in a series of aqueous suspensions formulated with colloidal silica, reactive alumina, and lime and aged for 1 year have been studied using 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (with and without cross polarization (CP)), solution pH, electron microscopy, and X–ray diffraction. As in earlier work dealing with the nature of C‐S‐H in the system CaO‐SiO2H2O2 two aluminum‐substituted C‐S‐H phases, having distinctly different anionic structures on the atomic level (Q2 versus Q1Q2), were found to extend into the system CaO‐Al2O3‐SiO2‐H2O2 X‐ray diffraction patterns of the two aluminum‐containing C‐S‐H phases are nearly identical, suggesting that their intermediate‐range order is very similar, but MASNMR spectra show that these two phases have markedly different silicate structures on the atomic‐level scale. Both C‐S‐H structures can accommodate approximately 5 mol% of Al2O3 in tetrahedral and possibly octahedral coordination as well.