Nylon-6 (N6)/nylon-6,6 (N66) blend membranes were prepared from N6/N66 blend solutions in calcium chloride-methanol mixture of the concentrates in order to investigate the possibility of porosity control. During the pore formation process, which took place at 10 and 20 degrees C by adding water onto the blend solution surface in a cylindrical glass bottle, a four-phase structure of solid, gel 1, gel 2 and solution (in order from the top) was observed clearly. With increasing the ratio of N66 in solution, the semitransparent grey colour of gel 2 hardly changed, but the colour of gel 1 changed gradually from semitransparent grey to white. In the boundary region between gel 1 and gel 2, microphase separation between N6 and N66 was predicted. The macroscopic pores in membranes prepared at 10 and 20 degrees C were almost spherical and with increasing the ratio of N66, the pore size decreased abruptly between blend ratios of N6 and N66, 1:0 and 2:1, and then gradually approached that of N66. when the humidity in the concentration conditions was increased from similar to 35 to similar to 80% r.h. at 20 degrees C, the pore size increased by about 46% in diameter.