The use of liquid-liquid thermally induced phase separation (TIPS) to produce anisotropic cellular flat sheet membranes was investigated fur the model system isotactic polypropylene (iPP) and diphenyl ether (DPE). Anisotropic structures were obtained experimentally by evaporating diluent from one surface of a polymer diluent solution prior to phase separation. The effects of convective air flow above the exposed surface were studied. The effects on membrane cell size and extent of anisotropy were studied using scanning electron microscopy (SEM); the effects on the iPP concentration profile within the nascent membrane at the time of phase separation were predicted using a model developed in this work. The extent of anisotropy in the membrane and the predicted concentration profile were compared as one measure of the validity of the model. The theoretical and experimental weight loss during evaporation were also compared as a further measure of the validity of the model. The effects of polymer molecular weight on membrane cell size and cell size profile as well as weight loss were also studied.