Cation exchange membranes, such as Nafion(R) 117, are widely used in fuel cells and electrolyzers. The conductivity and hence the performance of these membranes depends strongly on the amount of free water contained within the membrane. The present paper uses the coaxial probe method, developed by the authors, to examine the average and local conductance of Nafion(R) 117 under different degrees of humidification. The ion transport characteristics of the membrane are also studied under conditions of differential humidification where opposite sides of the membrane are held at different humidity values. The results offer a new approach to membrane characterization and further insight into the role of water content in the transport of H+ ions across the membrane in fuel cells.