Organosilica membranes were fabricated under controlled sol-preparation conditions such as the ratio of triethoxysilane (TRIES) and vinyltrimethoxysilane (VTMS) and the hydrosilylation temperature. The single-gas permeation properties and hydrothermal stability of organosilica membranes were evaluated to clarify the relationship between hydrothermal stability and organosilica structure. Pt-catalyzed and thermally cured hydrosilylation was applied to evaluate the effect that hydrosilylation temperature exerts on the properties of membranes. Organosilica membranes (Pt-catalyzed hydrosilylation at 40 degrees C) showed H-2 permeance of approximately 10(-6) mol m(-2) s(-1) Pa-1 with H-2 selectivity (H-2/CH4:15, H-2/CF4:950) at 500 degrees C, and were stable under an oxidative atmosphere at 500 degrees C. The organosilica network size derived by thermal curing at 500 degrees C was smaller than that by Pt-catalyzed hydrosilylation, even though the units (Si-C-C-Si, Si-O-Si) were the same. Hydrosilylation reactivity derived by thermal curing (500 degrees C, N-2) strongly depended on the TRIES/VTMS (=H/V) ratio in the SQ sol, and an H/V ratio of 1.25 showed a higher level of hydrosilylation reactivity. Its hydrothermal stability was better than that of amorphous silica membranes, due to the incorporation of Si-(CH2)(2)-Si units in the networks via hydrosilylation, based on the decreased ratio of He and H-2 permeance, the He/H-2 permeance ratio, and the activation energy before/after steam treatment. (C) 2015 Elsevier B.V. All rights reserved.