Membranes were prepared by the direct and isothermal immersion of polyamide solutions in a formic acid/water bath. A crystalline polycaprolactam homopolymer, nylon 6, and a largely amorphous terpolymer of nylon 6, nylon 66, and nylon 610 were precipitated from Solutions to form complex morphologies on the top and bottom Surfaces and cross sections of the membranes. Terpolymer membranes exhibited the characteristics of a liquid-liquid phase-separation process. According to the conditions of the solution and bath, nylon 6 precipitated to form membranes that showed dominance of crystallization or liquid-liquid phase separation. By precipitation from a solution containing a high concentration of a nonsolvent into a bath containing a hi-h concentration of formic acid, skinless nylon 6 0 microporous membranes were formed. (c) 2005 Wiley Periodicals, Inc.