The membrane formation by the phase inversion process was studied by coagulating a polysulfone/N-methyl-2-pyrrolidone solution with water vapor as a coagulant. The phase separation occurred when the relative humidity in the membrane casting atmosphere was higher than about 65%. The pore size was strongly affected by the relative humidity as well as the concentration of the polymer solution. It increased as both the relative humidity and the polymer concentration were decreased. The membranes produced showed a uniform structure composed of closed pores. The pure water flux measurement confirmed the closeness of the pores. The information on the late stage phase separation was obtained in situ by an optical microscope due to the slow phase separation. The pores seemed to grow very much at the late stage by coarsening which was observed to occur mainly by coalescence of polymer-lean droplets. As the relative humidity was lower, the coarsening continues longer ending up to a larger droplet size. The coarsening seems to enhance the interconnectivity of pores when the polymer concentration was low enough.