The equilibrium phase behavior of water (nonsolvent)-DMF (solvent)PVDF system at 25 degrees C was investigated via both theoretical and experimental approaches. Using binary interaction parameters, chi(ij), obtained previously, the theoretical phase boundaries were computed and were found to match closely the measured binodal and crystallization-induced gelation data. Membranes were prepared using the isothermal immersion-precipitation processes in various dope and bath conditions. The formed membranes demonstrated a broad spectrum of morphologies: At one extreme, asymmetric structure was obtained featuring a continuous tight skin and a sublayer composed of parallel macrovoids and cellular pores; at the other limit, skinless micro-porous membrane was produced with spherical particles packed into a bi-continuous structure. The crystalline characters of PVDF gels and membranes were examined by small angle light scattering, scanning electron microscopy, and differential scanning calorimetry techniques. In addition, diffusion trajectories and concentration profiles in the membrane solution before precipitation were calculated for the immersion process. These results predicted reasonably the various morphologies observed in the membranes.