This work investigated the preparation of composite membranes by simultaneous casting of polyurethane and polyethersulfone solutions, to form the top and the support layers, respectively. These membranes are intended to be used for propylene/propane separation. Polyvinylpyrrolidone was used as an additive in the corresponding solution to obtain a support with low transport resistance. The synthesis conditions were related to the morphology of the resulting membranes. Polyurethane concentration, nature of the solvent and exposure time before immersion into the precipitation bath were primarily investigated, in addition to the influence of additives, such as lithium nitrate and formamide.