Organic/inorganic hybrid silica membranes were prepared from 1,1,3,3-tetraethoxy-1,3-dimethyl disiloxane (TEDMDS) by the sol-gel technique firing at 300-550 degrees C in N(2). TEDMDS-derived silica membranes showed high H(2) permeance (0.3-1.1 x 10(-6) mol m(-2) s(-1) Pa(-1)) with low H(2)/N(2) (similar to 10) and high H(2)/SF(6) (similar to 1200) perm-selectivity, confirming successful tuning of micropore sizes larger than TEOS-derived silica membranes. TEDMDS-derived silica membranes prepared at 550 degrees C in N(2) increased gas permeances as well as pore sizes after air exposure at 450 degrees C. TEDMDS had an advantage in tuning pore size by the "template" and "spacer" techniques, due to the pyrolysis of methyl groups in air and Si-O-Si bonding, respectively. For pore size evaluation of microporous membranes, normalized Knudsen-based permeance, which was proposed based on the gas translation model and verified with permeance of zeolite membranes, reveals that pore sizes of TEDMDS membranes were successfully tuned in the range of 0.6-1.0 nm. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 2755-2765, 2011