Organosilica membranes for gas separation were prepared by plasma-enhanced chemical vapor deposition (PECVD) using three different types of silicon precursors: hexamethyldisiloxane (HMDSO), trimethylmethoxysilane (TMMOS), and methyltrimethoxysilane (MTMOS). Based on gas permeation measurement, the MTMOS-derived membrane showed the highest He/N-2 selectivity, followed by the TMMOS-derived and HMDSO-derived membranes. FT-IR characterization indicated that the HMDSO-derived membrane had the highest content of methyl group and the lowest Si-O-Si, while the methyl group content for the MTMOS-derived membrane was the lowest and Si-O-Si was the highest. These results suggest that the pore size of organosilica membranes could be tuned by changing the chemical structure of the silicon precursor. The MTMOS-derived membrane was further heat treated to determine the effect of thermal annealing on gas-permeation properties. The gas permeances were drastically improved by the thermal annealing. After heat treatment at 500 degrees C, the membrane showed a high H-2 permeance of 6.5 x 10(-7) mol/(m(2) s Pa) with a H-2/SF6 selectivity of 410 at 200 degrees C, arid 5.6 x 10(-7) mol/(m(2) s Pa) with a H-2/SF6 selectivity of 360 at 50 degrees C. (C) 2015 Elsevier B.V. All rights reserved.