Kinetics of phase separation of polymer solution layers induced by nonsolvent vapor was investigated. Polymer solutions were prepared with polysulfone (PSF) and N-methyl-2-pyrrolidinone (NMP) as a solvent. The phase separation was induced by exposing the solution layers to controlled water vapor. Using time-resolved small angle light scattering and phase contrast optical microscope, the kinetics of the phase separation was studied for several concentrations of polymer solution films. The phase separation took place as the solution layers absorbed nonsolvent (water) vapor. As the phase separation proceeds, the scattering halo was observed, which implies that the phase separation occurs via spinodal decomposition. Bicontinuous network structure, which is a characteristic morphology of the spinodal decomposition, was confirmed by optical microscopy. The analysis of the early stage of phase separation was based on Cahn-Hilliard theory and mutual diffusion coefficients were evaluated. The late stage of phase separation was analyzed by means of power law (q m โผ t -โฃ ). In the late stage of the phase separation, spontaneous pinning of phase domain was observed. Finally, when the phase-separated solution layers were coagulated in the water bath, it was found that the correlation length at pinning regime is proportional to the pore size of the resulting material.