Sol-gel polymerisation routes have been devised to produce microporous silica membranes for CO2/N-2 gas separation. The micropores are produced by the thermo-destruction of organic ligands (so-called templates) covalently attached to the silica network. The thermal stability and structural evolution with temperature of these materials has been characterised with TGA, differential thermal analysis (DTA), FTIR, C-13 CP MAS NMR and Si-29 MAS NMR investigations. The permeability Of CO2, N-2 and He was measured through the membranes, as a function of pressure (front and back), temperature and time. CO2 was found to permeate preferentially to He in optimum membranes despite its larger molecular diameter. Also, an inverse relationship was found between permeability and temperature. Together this provided strong evidence that gas transport is governed by surface diffusion. The permeability of N-2 and He was found to be invariant with time, whilst a characteristic permeability time curve was noticed for CO2 (CO2 permeability initially increased, before declining). The decrease in permeability has been attributed to CO2 chemisorption, and is linked to the surface diffusion mechanism. Surface diffusion is enhanced by the incorporation of phenyl ligands within the siloxane network, but this is at the expense of accelerated adsorption (the phenyl ligand acts as a weak catalyst).