The influence of added short-chain amines, n-butylamine, BA, and n-octylamine, OA, on the mesophase formation of surfactant-silicate composite structures in a room-temperature synthesis has been investigated in situ by synchrotron small-angle X-ray scattering. For BA a marked decrease in the d spacing of the hexagonal phase was observed with increasing amine concentration, while a slight increase in d spacing was initially observed for OA. However, the d spacing of the hexagonal phase went through a maximum with increasing concentration. Furthermore, two coexisting hexagonal phases or a purely lamellar phase was observed if the concentration of OA was increased further. The results are discussed in terms of two competing mechanisms of amine incorporation: solubilization within the CTAB-silicate mesophase and competitive adsorption of the amine to the silicate.