Silica-based membranes were prepared by atmospheric-pressure plasma-enhanced chemical vapor deposition. The effects of membrane preparation parameters such as thermal annealing temperature and precursor on their gas permeation properties were investigated by assessing the temperature dependence of their gas permeances. Thermal annealing was effective for the improvement of membrane performance owing to the formation of permselective pores via partial decomposition of organic components in the plasma-deposited layer. The membrane prepared using hexamethyldisiloxane (HMDSO) as a silicon precursor and annealed at 400 degrees C in N-2 showed a high H-2 permeance of 1.6x10(-6) mol m(-2)s(-1)Pa(-1) with H-2/N-2 and H-2/SF6 permeance ratios of 53 and 1,800, respectively, at 300 degrees C. High carbon-content structures were obtained by plasma-deposition of HMDSO mixed with a second precursor (1,5-cyclooctadiene) or a carbon-rich silicon precursor (triethylsilane). Gas permeation measurements revealed that a higher the carbon content in the plasma-deposited layer leads to a higher activation energy of permeation.