The diffusional processes proceeding in the intercalation electrode on applying a potential step are theoretically treated. The theory of chronovoltammetry is shown to require the necessary account of the contribution of the surface solid-state film to the overall diffusion resistance of the electrode. For the case of small deviations from equilibrium, analytical solutions of the diffusion problem have been obtained. Ignoring the retarding contribution of the surface film is shown to bring about an error into the diffusion coefficient of lithium in its alloys and intercalates. The theory has been experimentally verified with LixC6 electrodes of various compositions made of pyrocarbon films on a nickel backing as examples. The diffusion coefficients are 10(-11) -10(-9) cm(2) s(-1), depending on composition.