A METHOD IF DISCLOSED FOR THE PREPARATION (BY THE UTILIZATION OF A PROPER SOLVENT SYSTEM) OF DRY ASYMMETRIC MEMBERANES COMPRISING A POROUS LAYER OF INTERCONNECTED CRYSTALS OF POLYMER MATERIAL. MEMBRANES OF MANY POLYMER MATERIALS MAY BE OPTIONALLY PREPARED EITHER WITH OR WITHOUT A DENSE SURFACE LAYER AS ONE FACE THEREOF. IN EITHER CASE THE POROUS LAYER IS STRUCTURED WITH GRADED POROSITY. A THREE-COMPONENT CASTING SOLUTION SOLUTION IS PREPARED CONTAINING THE POLYMER, A FIRST GOOD VOLATILE SOLVENT FOR THE POLYMER AND (RELATIVE TO THE FIRST SOLVENT) A POOR LESS-VOLATILE SOLVENT FOR THE POLYMER, WHICH IS MISCIBLE WITH THE GOOD SOLVENT. A MEMBRANE IS CAST, ALLOWED TO DESOLVATE FOR A SHOT TIME AND IS THEN IMMERSED IN A LEACHING AGENT, THAT IT MISCIBLE WITH BOTH THE AFOREMENTIONED SOLVENTS BUT IS A NON-SOLVENT FOR THE POLYMER. THE MEMBRANE IS THEN PERMITTED TO DRY.