An analysis of data from scanning electron microscopy (SEM), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) for two series of polyurethane membranes is presented. Membranes were prepared by direct immersion of the casting solution of polyurethane, dissolved in dimethyl formamide at different temperatures, into either water or 1-octanol bath at 25 °C. Depending on the temperature of polyurethane dissolution, a gradual variety in the membrane structure was observed by SEM. As the temperature of polymer dissolution was increased, the membrane structure changed from dense to cellular or particulate morphologies. On the basis of the GPC and DSC results, polyurethane molecular weight decreased but the degree of microphase separation caused by clustering of some of the soft and hard segments into separate domains in the membrane increased with increasing the temperature of polymer dissolution. It is thus proposed that the change in membrane structure is due to the variation of molecular weight of polyurethane, which in turn can change polymer chain mobility during membrane formation. In addition, the extent of microphase separation was described and related to the particulate morphology when 1-octanol was used as the nonsolvent.