Chlorinated poly(vinyl chloride) (CPVC)/poly(vinyl pyrrolidone) (PVP) membranes were prepared by using the solvent system tetrahydrofuran (THF)/n-butyl alcohol (n-BA) to investigate the possibility of pore size and pore-size distribution control. The coagulation of CPVC/ PVP solution was induced by the exposure to water vapor at 25 (+/-0.5)degreesC. The average pore diameter, d(p), and the size distribution of pores on the surface of the membrane were quantified through the image analyzer from the images visualized by field emission scanning electron microscope (FE-SEM). Surface pore size and distribution of the prepared CPVC/PVP membrane were strongly affected by the relative humidity (RH) in the environment and the content of PVP used as an additive. Particularly, in the case of CPVC membrane without PVP, the mean pore size was 0.15-0.2 gin, depending on the RH. The pore distribution became broad with the increase of the RH. The membranes had open pores as confirmed by the hydraulic permeation experiment. In addition, the water flux and membrane resistance (R-m) were greatly affected by the composition of polymer solution and the RH.