The work was aimed at investigating the ohmic drop in ion-exchanging resin beds in EDI cells, by using two techniques. The case of CuSO4-H2SO4 solutions was treated here, using Dowex resins of various degrees of cross-linking. Impedance measurements in a dedicated cell filled with resin packed beds were interpreted by using a model for the conductivity of two-phase media. Secondly, the voltage recorded in a small EDI cell for continuous treatment of copper solutions, led to the bed conductivity, using a model for potential distribution in the cell. The consistency of the two techniques was discussed, and side electrical phenomena in the EDI cell were shown to be the source of an unexpectedly significant increase in the cell voltage.