The electrical conductivity of packed beds containing macroporous Duolite C-25 polystyrene sulfonate cation exchanger equilibrated with aqueous solutions of a single electrolyte (NaCl or HCl) has been measured over a range of electrolyte concentrations. The results are interpreted in terms of a two-phase model for the resin particle involving (1) the resin matrix, with the conventional properties of polystyrene sulfonate cation exchanger, and (2) a macropore dispersed phase which is assumed to have the same conductance as the solution outside the particle. The resin particle conductivity is calculated from the Bruggeman equation using measured bed and interstitial solution conductivities. The external void fraction is assumed to be 0.36. Applying the Bruggeman equation to the two-phase model, the resin matrix conductivity can be calculated from the previously calculated resin particle conductivities and the solution phase conductivities. A nearly constant resin matrix conductivity (which seems realistic for the electrolyte concentrations studied) is calculated for a macropore volume fraction of approximately 0.15. The experimental value of about 0.10 is in good agreement.