Carbon membranes with 0.2 and 1.0 μm pore sizes are commercially available for liquid microfiltration applications. These membranes may be modified for gas separation applications by providing a gas separation layer with pores in the 1 to 10 nm range. With such pores, gases are separated by Knudsen diffusion with an individual gas species permeation rate inversely proportional to the ratio of the square root of the molecular weight of the permeating species. This paper describes some of the techniques used for depositing a suitable layer starting with various organic polymeric precursors. The in situ polymerization technique was found to be the most promising, and pure component tests with membrane samples prepared with this technique indicated Knudsen diffusion behaviour. The gas separation factors obtained by mixed-gas permeation tests were found to depend strongly on gas temperature and pressure indicating significant viscous flow at high-pressure conditions.