The pH adjacent to the surface of a film-covered electrode is generally a difficult quantity to measure. Consequently, we have developed a model to calculate the concentration distribution of a reacting species in a porous film on an electrode surface. Reactions at both the electrode surface and at the film surface within the pore structure are considered. Effects of pore geometry, diffusion rate and reaction rate on concentration distribution are computed from a one-dimensional model. Use of the model in predicting hydroxide ion concentration near the surface of a passivating zinc electrode is demonstrated.