Interpenetrating polymer networks (IPNs) of polyurethane (PU) and polystyrene (PS; 90/10 and 75/25) were synthesized by the condensation reaction of castor oil with methylene diisocyanate and styrene, with benzoyl peroxide as an initiator. The IPN membranes were characterized for physicomechanical, optical, and X-ray diffraction properties. Computer-simulated concentration profiles of aqueous salt solutions through PU/PS IPN membranes were generated with Fick's second-order differential equation, and the results were examined in terms of diffusion anomalies. (C) 2003 Wiley Periodicals, Inc.