Microporous silica membranes with high permselectivity are fabricated by atmospheric-pressure plasma enhanced chemical vapor deposition (AP-PECVD) using hexamethyldisiloxane as the precursor in plasma working gases of pure argon, and mixture of argon with oxygen or nitrogen. A silica membrane grown using plasma composed of a mixture of argon and nitrogen displays highly efficient gas separation, with selectivities for He/N-2 and He/SF6 of 196 and 820, respectively, and He permeance of 1.1x10(-7) mol m(-2) S-1 Pa-1 at 50 degrees C. Characterization of the membranes by FTIR and X-ray photoelectron spectroscopies reveals a relatively high concentration of carbon remains in the membrane grown using a mixture of argon and nitrogen. Annealing at elevated temperature after plasma deposition improves the permselectivity of the membranes. After annealing at 300 degrees C, the permeance of He at 50 degrees C increased to 4.0x10(-7) mol m(-2) s(-1) Pa-1 with no marked decrease of selectivity (He/N-2 =98, He/SF6 =770). The annealed membrane also exhibits remarkable permselectivity for CO2, showing selectivities for CO2/N-2 and CO2/CH4 of 46 and 166, respectively, with CO2 permeance of 1.9 x10(-7) mol m(-2) s(-1) Pa-1 at 50 degrees C. AP-PECVD shows great promise to fabricate microporous silica membranes highly permselective for gas separation.