Network size control was evaluated for micro porous membranes derived from triethoxysilane (TRIES) that contains highly reactive Si-H groups. It was possible to control the concentration of the Si-H groups via the conditions of calcination (temperature, atmosphere). Si-H groups remained within their network structure when the TRIES membrane was calcined at 350 degrees C under a N-2 atmosphere, and had a loose network structure (H-2 permeance: 5.40 X 10(-7) mol m(-2) s(-1) H-2/CH4 selectivity: 36). When calcination at high temperatures converted the Si-H groups to Si-O-Si groups, the TRIES membrane showed a high level of separation performance (H-2 permeance: 2.34 X 10(-7) mol m(-2) s(-1) pa(-1), H-2/CH4 selectivity: 590) due to a densification of the network structure. Compared with conventional microporous silica membranes, a TRIES membrane with Si-H groups showed hydrophobic properties, but water vapor was adsorbed and/or capillary-condensed in the microporous structure, and permeation blocking for He molecules was observed at temperatures below 150 degrees C in the presence of saturated water vapor at 25 degrees C. Hydrophobic Si-H groups improved the hydrothermal stability at 300 degrees C, but depending on the partial pressure of the steam, the reaction between Si-H groups and water vapor degraded the hydrothermal stability of the TRIES membranes.