The SO2 transport properties of Nafion(R) and sPEEK membranes were measured using an electrochemical reaction cell to investigate their application in the electrochemical hybrid sulfur process. The permeability of SO2 in the membranes was determined from a combined theory based on Faraday's law and Fick's law where the electrochemical reaction rate of SO2 in the downstream membrane is the same as its diffusion flux through the membrane. Both Nafione and sPEEK membranes show higher SO2 diffusion coefficients at higher temperatures. For sPEEK membranes, increasing the degree of sulfonation resulted in increasing permeability, as more water was imbibed in the membranes with higher degrees of sulfonation. Activation energy was extracted from the temperature-dependence of the diffusion coefficients for both membranes. The sPEEK membranes exhibited similar diffusion coefficients to those of Nafion(R), even at high sulfonation degrees of 70%. Besides SO2 permeability, proton conductivity and mechanical properties were measured for comparison between the 2 polymer membranes. Although the proton conductivity of the sPEEK was slightly lower than the Nafion(R) membrane, it was very competitive considering its higher mechanical strength and much lower cost. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.