Poly(imide siloxane) (PIS) precursors have been synthesized in different compositions and appraised in their ability to form carbon membranes containing silica (C-SiO2). During inert pyrolysis, the imide domains in the PIS were transformed into a carbon-rich phase conferring a molecular sieving capability for molecular pairs of small gases, such as H-2/N-2, He/N-2, CO2/N-2, and O-2/N-2. The siloxane domains in the PIS were converted into a silica-rich phase, which provided further gas pathways. The siloxane chain length in the PIS influenced the gas permeation characteristics of the C-SiO2 membranes. A change in the siloxane chain length of the polydimethylsiloxane (PDMS) segment in the PIS precursor directly influenced the gas permeation and separation properties of the C-SiO2 membranes. For a constant volume fraction of PDMS moieties, a longer siloxane chain in the PIS led to a drastic increase in gas permeability and a reduction in gas selectivity of the C-SiO2 membranes. Moreover, the diffusion coefficients of selected gases were also affected by the silica phase, which was embedded in the continuous carbon matrix. The contribution of mobility (diffusion) selectivity to the overall selectivity decreased with volume fraction of PDMS moieties in the PIS precursor. Mixed gas separation experiments were conducted at 25 degreesC and at a feed pressure of 5 atm (1 atm = 101325 Pa) using feed gas mixtures, such as O-2/N-2 (71%/29%) and CO2/N-2 (15%/85%). For mixed gas separation, carbon-coated microporous alumina tubes were fabricated by dip-coating alumina tubes into siloxane-containing poly(amic acid) (SPA) solutions followed by pyrolysis at temperatures up to 600degreesC. The separation properties of the composite membranes showed good agreement with the results obtained from single pure gas permeation experiments. (C) 2004 Elsevier B.V. All rights reserved.