Abstract The influence of different coating procedures on the transport and separation properties of composite gas separation membranes was studied. The composite membrane (CM) comprises a coating material (silicone rubber) in occluding contact with an asymmetric polysulfone flat membrane (AM). Permeabilities, ideal separation factors and structure of the CM were found to depend strongly on the evaporation time, volume and concentration of the coating solutions. Gas permeation experiments (H 2, N 2, CO 2, CH 4) indicated that gas permeabilities decreased rapidly with an increasing amount of coating. A maximum in the H 2/N 2, H 2/CH 4 ideal separation factor was reached as the amount of coating was increased. Scanning electron microscopy (SEM) studies revealed the presence of a dense isotropic layer of coating material above an anisotropic layer comprised of a mixture of polysulfone nodules and silicone material. The results showed that the CM prepared with a concentration of 6% silicone solution and contact time of 1 min has the best gas separation performance.