Defect-free skinned asymmetric gas separation membranes were prepared by a dual bath coagulation method that is a wet/wet phase inversion technique. The membranes were cast from a polysulfone/N,N-dimethyl-acetamide solution. In two sequent nonsolvent baths, the first bath using iso-propanol (IPA) leads to the formation of a dense skin top layer and the second bath using water makes the actual polymer precipitation. The top skin layer thickness was governed by changing the immersion time of the first IPA bath. We suggest that the growth rate of the skin layer is to be determined by a diffusion process.