Aliphatic polyamides (Nylon-66 and a Nylon-6, -66, -610 terpolymer) were isothermally precipitated from formic acid solution by immersing in an aqueous nonsolvent bath or a solvent/nonsolvent mixture. Depending on the composition of the polymer solution and nonsolvent bath, phase separation by nucleation and growth can be initiated for a liquid-liquid phase separation process, a crystallization process or a combined process. Under certain conditions, crystallization of Nylon-66 results in a membrane with a uniform skinless microporous structure that was rapidly wetted by water. In contrast, liquid-liquid phase separation produces a polyamide film with largely unconnected cellular voids that is as a result not wetted by water.