Periodic mesoporous organosilicas (PMOs) synthesized by hydrolysis and condensation of bridged silsesquioxanes (RO)(3)SiR'-Si(OR)(3) in the presence of surfactant template are an emerging family of novel functional materials. In this study, the synthesis of ethylene-bridged PMOs with a highly ordered pore structure was demonstrated by using a nonionic surfactant P123 as a template under acidic conditions. Emphasis was made at the influence of SiO2/P123 ratios on the structural properties such as type of pore structure, the presence or absence of micropores and macropores, and pore volume. It was observed that an increase in SiO2/P123 ratios led to a gradual increase in the microporous volume. In the mean time, a structural transition from an open-channel pore structure to a cage-like pore structure was observed as the SiO2/P123 ratios were increased. (C) 2004 Elsevier B.V. All rights reserved.