Ionic diffusion in polypyrrole films has been studied with ex situ Rutherford backscattering spectrometry. Polypyrrole films were deposited from aqueous electrolytes containing one of the following salts : sodium perchlorate, sodium xylene-sulfonate onate, or poly(sodium 4-styrene-sulfonate). The ionic content of the films was determined as a function of potential (steady-state results) and as a function of the charge passed during a potentiostatic oxidation or reduction. Results were also obtained for films which were ion exchanged by potential cycling in alkali halide electrolytes. Diffusion coefficients on the order of 10(-10) cm(2)/s were obtained for the ClO4-, Na+, and Cs+ ions. The relative mobilities of the ions are in agreement with their size, assuming the alkali metal cations retain their hydration sheaths within the polymer matrix. Thus perchlorate was found to have the highest diffusion coefficient, while the poly(styrene-sulfonate) anion was found to be the least mobile.