Impedance spectra were recorded for glassy carbon electrodes coated with poly(styrenesulphonate) and Nafion(R) films containing Os(2,2＇-bipyridine)(3)(3+/2+) as a function of the concentration of incorporated redox sites and applied potential. Experimental data were interpreted in terms of equivalent circuits based on finite transmission lines. Comparisons of the odor capacitances derived from cyclic voltammetry and from impedance spectroscopy showed that only a fraction of the total number of redox sites present in Nafion(R) coatings was active during impedance measurements. The concentration dependence of the apparent diffusion coefficient for electron propagation through Nafion(R) films suggested the presence of charge percolation.