Heterogeneous hollow fiber membranes with cation exchange functionality are prepared using a wet spinning technique. The spinning dope solutions are prepared by dispersing finely ground cation ion-exchange resin (CER) particles in an N-methyl pyrrolidone solution of polysulfone (PSF). The polymer solution is shaped into a hollow fiber by forcing the solution through a tube-in-orifice spinneret followed by phase inversion in a water bath. The ultimate hollow fiber membranes consist of cation exchange resin particles (size: less than 30 ¿m) entrapped inside a microporous matrix structure. The weight ratios CER/PSF of the dope solutions studied are 0/100, 20/80, 30/70, 40/60, and 50/50. A careful control of the pore size is important for the membrane performance. Because of the porosity of the matrix the resin particles maintain their functionality. In diffusion dialysis using NaOH, NaCl, and Na2SO4 solutions, it was observed when increasing the resin content of the hollow fibers, the flux ratio of NaOH versus salts increases till a resin content of 40 wt.% is used. The flux ratio between alkali and salts is very low for the membranes which contain 50 wt.% CER due to their rather large pores in the matrix structure. By heating the hollow fiber membranes containing 50 wt.% CER at 200 °C, the flux ratio of alkali and salts increases with increasing heat treatment time. Heating for 10 min increases the flux ratio between NaOH and Na2SO4 from 3.3 to 102.