New very stable composites prepared by deposition of resorcinol-formaldehyde polymer (RF-) or carbon (C-) xerogels into walls of commercial porous tubular ceramics (TiO2-ZrO2 and αAl2O3-γAl2O3) were obtained by a sol-gel process followed by a drying and a pyrolytic (only for C-xerogel/ceramic composites) step. They were characterized by nitrogen adsorption-desorption, SEM and XRD, and tested for gas (H2, CH4, CO2 and CO) separation applications. Additional morpho-structural information about the open-interconnected ultramicropore structure of composites was found by gas permeation investigation. Interesting results for H2 permeance was obtained especially for RF-polymer/ceramic composites respecting Knudsen diffusion mechanism of gas permeance: H2 > CH4 > CO > CO2. The coexistence of Knudsen and surface diffusion mechanisms were confirmed.