Electrical impedance spectroscopy (EIS) techniques were used to gain insight into BSA fouling of the Neosepta CMX and AMX ionexchange membranes (IEMs). EIS characterizations were made at concentrations above 0.1 M KCl because the conductance of the IEMS was higher than that of bulk solutions of concentration below 0.1 M KCl. Spectra, expressed in terms of dispersions of the conductance and capacitance with frequency, provided an enhanced indication of IEM fouling during separation processes. Bulk conductance measurements of the solution alone, membrane immersed in solution and fouled membrane immersed in solution showed good agreement with general theoretical predictions. Strong dispersions in capacitance were observed below 1 kHz for each of these configurations. Differences in the dispersions arising from fouling were identified by subtracting the impedance of the solution from those of unfouled and fouled IEMs in solution. The conductance and capacitance dispersions of fouled IEMs decreased with the accumulation of the BSA fouling layer on the surface. (C) 2004 Elsevier B.V. All rights reserved.