The reduction of Cr(VI) to Cr(III) on polypyrrole-modified glassy carbon electrodes in 0.1M H2SO4 was studied by rotating disk electrode voltammetry. The Cr(VI) substrate concentration and the polypyrrole film thickness were used as the process variables for these experiments. The data were analyzed within the framework of the theory of Andrieux, Saveant and coworker for polymer-modified electrodes. However, the permeation of Cr(VI) into the microporous polypyrrole matrix was facile, such that direct reduction of Cr(VI) at the support electrode/polymer film boundary was also taken into account. At electrode potentials at which the polypyrrole matrix was catalytically active, the cross-reaction between the reduced polymer sites and Cr(VI) was shown to be the rate-limiting step. At more negative potentials, the electronic conductivity of polypyrrole was degraded such that charge percolation through the polymer matrix limited the catalytic reaction rate.