Mesoporous silicas with variable pore size and two coexisting domains were synthesized by mixed systems of cationic and nonionic surfactants. The pore sizes of silica were tuned by modification of a mixed surfactant of cetyltrimethylammonium bromide (CTAB) and polyoxyethylene (10) cetyl ether (C16EO10) in acidic conditions. Various factors such as total surfactant concentration, molar composition, and the amount of acid affect the silica mesophase. The control of the pore dimension was achieved by changing the ratio of nonionic to cationic surfactants in reactant gel mixtures. At small ratios of C16EO10, an ordered. array based on CTAB templates was obtained. However, at high ratios, C16EO10-based silica of high d spacing was observed. At higher amounts of acid, biphasic regions, where the silica mesophase separated, each internal structure corresponding to its mother structure, were observed. This may be attributed to the preferential binding between C16EO10 and hydrolyzed products of tetraethyl orthosilicate, which was promoted by the hydrogen bonding accelerated by the amount of acid.