Porous silica membranes were formed by a sol-gel technique on a gamma-alumina-coated alpha-alumina tube using sols which were prepared from tetraethoxysilane (TEOS) and octyl-, dodecyl- or octadecyltriethoxysilane. The alkyltriethoxysilanes were used as the template to control the pore size. The effect of the length of alkyl groups on pore structure, as well as the permeation properties of the silica membranes were then investigated. The molar ratio of water to total alkoxides in the sols, x, strongly influenced the pore structure. A silica membrane which was prepared with octadecyltriethoxysilane contained mesopores and cracks, but silica membranes prepared with octyl- and dodecyltriethoxysilanes were defect-free. Permeations to gasses with a variety of molecular sizes showed that the majority of micropores were in the 0.3-0.4 nm range for the silica membranes which had been calcined at 600 degrees C. These micropores were basically formed by the decomposition of alkyl chains in the templates, while the mesopore structure was achieved during the gelation step.