The applicability of the thermally induced phase separation (TIPS) process to the production of anisotropic membranes was investigated. To induce an anisotropic structure, diluent was evaporated from one side of the polymer-diluent melt-blended, thereby creating a concentration gradient in the nascent membrane prior to inducing phase separation. The system used to prepare these membranes was isotactic polypropylene (iPP) in diphenyl ether. The resulting membrane structures showed that this evaporation process was useful in producing anisotropic structures. The effects of evaporation time and initial polymer concentration on the anisotropic membrane structure were investigated. The evaporation process was analysed by solving appropriate mass transfer and heat transfer equations. The agreement between the calculated results and the experimental data on the membrane weight loss and the membrane thickness was satisfactory. The membrane structures are discussed in detail based on the calculated polymer volume fraction profiles in the membranes.