The effects of the addition of two different types of cosurfactants, n-alkylamine and n-alkyl alcohol, on he phase behavior of silicate-surfactant mesophases and the pore size of the calcined materials have been investigated using X-ray diffraction, N-2-sorption, TG-MS, and solid state H-1 and C-13 NMR spectroscopy. The cosurfactants were added to a standard room-temperature synthesis, which results in a MCM-41 type, hexagonally ordered material in the absence of cosurfactant. Their addition led either to an increase or a decrease of the pore size and d spacing of the hexagonal mesophase, depending on the nature of the polar headgroup and on the chain length of the cosurfactant. For high cosurfactant/surfactant ratios, however, a transition to a lamellar phase was usually observed. The d spacing and the pore size of the hexagonal mesophase can be decreased considerably down to the supermicropore range by addition of butylamine without a marked decrease in the long-range order of the material. The addition of alcohol, on the other hand, resulted in an increase in the d spacing of the hexagonal phase by up to 3 Angstrom. These results are discussed in terms of differences in the ability of the cosurfactants to interact with the silicate as well as cosurfactant-induced changes in the packing parameter.