A new dioxane-bridged alkoxysilane, 2,5-bis[2-(triethoxysilyl)ethyl]-1,4-dioxane (BTES-ED), was synthesized as a precursor of organically bridged silica membrane to investigate the effects of the 1,4-dioxane ring as a rigid and polar spacer. The monomer was polymerized by the sol-gel process to give the gels in film, powder, and membrane forms. Nitrogen adsorption measurement of the gel powder showed the type-I adsorption/desorption isotherm, suggesting that the gel possessed a microporous structure with pore size distribution ranging from approximately 0.8 to 1.6 nm. Reflecting microporosity, the membrane exhibited selective gas permeation properties (H-2/SF6 permeance ratio = ca. 1900) based on molecular sieving effects. Reverse osmosis experiments were conducted using a 2000 ppm NaCl aqueous solution at an operation pressure of 1.0 MPa, revealing water desalination properties with the water permeance of 1.84 x 10(-13) m(3)/m(2).s.Pa and the NaCl rejection of 98.5%. These results are discussed in comparison with the results of other bridged silica membranes reported previously. The 2,5-diethylene-1,4-dioxane bridging unit is rigid and polar, thereby contributing to improving gas and water separation performance.