Solid-state Si-29 cross-polarization magic angle spinning nuclear magnetic resonance (Si-29 CP/MAS NMR) was used to investigate the microstructurre of SiO2/PEG-based proton conducting hybrid membranes, which were synthesized through acid catalyzed sol-gel reactions. Poly(ethylene glycol) (PEG) and 3-(triethoxysilyl)propyl isocyanate were used as the organic and inorganic phases, respectively. The silica phase was further modified with monophenyl triethoxysilane (MPh) to improve the mechanical rigidity of the membrane. Acidic moieties of 4-dodecylbenzene sulfonic acid (DBSA) were doped into the network structure at different levels to provide the hybrid membrane with proton conducting behavior. The effect of MPh content and the DBSA doping levels on microstructures was studied and the extent of condensation reaction was calculated. Finally, an attempt has been made to correlate the observed microstructures and physical properties such as water uptake, ion-exchange capacity, ionic conductivity, thermal, and mechanical behaviors of the hybrids. (C) 2003 Elsevier B.V. All rights reserved.