Composite membranes were prepared by mixing RuO2 center dot xH(2)O (with x = 0.0, 0.6 and 2.2) and polypropylene, an inert polymer matrix. The impedance of these homogeneous composite membranes was measured in 0.5 M H2SO4 over a frequency range from 1 mHz to 1 kHz. A three-dimensional model was devised to fit the experimental impedance spectra. It consists of a percolating network of homogeneous transmission lines to represent RuO2 center dot xH(2)O, and resistors to account for the polypropylene inert matrix. The model has four parameters that can be varied: P, C, Re and Ri, which are the volume fraction, capacitance, and electronic and ionic resistance of the active material, respectively. The model allows establishing a link between the macroscopic (and measurable) properties of a composite membrane and the microscopic properties of the constitutive materials. Accordingly, an upper limit to the DC proton conductivity of hydrous ruthenium dioxide was established: sigma(i) = 64, 27 and 10 mS.cm(-1) for x = 0.0, 0.6 and 2.2, respectively. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.010209jes] All rights reserved.