The interactions among polymers, solvents and gelating solutions have been investigated by measuring viscosity changes of polymer solutions, the heat of mixing between solvent and gelation medium. and gelating points in order to clarify selection criteria for solvent and gelating medium for the preparation of polysulfone (PS-1700) membranes. It was found that the solvent of a casting solution which is suitable for the preparation of the asymmetric membrane, could be determined by measuring the viscosity changes of the polymer solution (vol%), and that the appropriate combination of solvent and gelation medium could be determined by measuring the heat of mixing and the gelating points. In addition, it was concluded that the gelating points, which were measured at the initial phase separation by titration of a gelating solution into a dilute polymer solution (less than 1 vol%), could be described by K=C.gamma(n), in analogy with that of precipitate reactions. From the observation of the cross section of the membranes, it was concluded that a large n value was favorable for the preparation of membranes with a fine porous structure.