AbstractPoly(vinyl chloride) (PVC)/polyacrylonitrile (PAN) blend hollow‐fiber membranes were prepared by a phase‐inversion method with water as inner and outer coagulations. The influence of the compatibility of the two polymers on the formation of interfacial microvoids (IFMs) in the PVC/PAN blend hollow‐fiber membranes was investigated by the theory of thermodynamics and examined by Fourier transform infrared spectroscopy, dynamic mechanical analysis, viscometry, and scanning electron microscopy. All of the results show that good compatibility did not exist in the PVC/PAN blend systems; this led to the formation of IFMs between the two polymers. Also, the performance of the experimental results showed that the addition of PAN contributed to the enhancement of the permeability of the blend membranes; this laid the foundation for further study of PVC/PAN blend hollow‐fiber membranes with antifouling properties after hydrolysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012