The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at the high and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated. Atomic force microscopy and scanning tunnelling microscopy scans of the membrane surface suggested that the membrane pore diameters were approximately 5 Angstrom in diameter. Comparison between pure methane diffusivity through the membrane and those through nanoporous zeolites of various pore openings also suggested that the membrane pores diameters were in the range of 5-6 Angstrom. A sensitivity analysis indicated that the membranes contained a very narrow distribution of pore sizes.