A gas-phase sulfur dioxide (SO2) depolarized electrolyzer has been investigated for the large scale production of hydrogen. One issue that negatively affects electrolyzer operation is the diffusion of SO2 from the anode to cathode. A viable method to reduce crossover is to use a thin, low-permeability anode prelayer (LPAP) film with significant resistance to SO2 transport supported on a high proton conductivity membrane. However, it is difficult to measure the LPAP film's properties as this layer is not free standing. Hence, we developed a model that allows us to obtain the gas transport properties (i.e., diffusion coefficient and gas solubility) of a prelayer knowing the gas transport properties of the supporting membrane using the electrochemical monitoring technique (EMT). We experimentally verified our two-layer model via EMT experiments using membrane electrode assemblies (MEAs) made of single layers of Nafion 115 (N115) and Nafion 212 (N212), and an MEA of N115 and N212 laminated together; the N212 membrane served as a pseudo-LPAP film where our model was used to extract the N212 parameters knowing those of N115. We then applied our model to an MEA consisting of an LPAP film on a N115 membrane to obtain the LPAP film's SO2 transport properties. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.019210jes] All rights reserved.