The mass-transfer boundary layer adjacent to an electrode can contribute to the measured AC impedance of the electrode. Simple diffusion of the reactant across a stagnant diffusion layer can be analyzed exactly, but complications arising from homogeneous reactions, variable properties, or ionic migration can modify the impedance, and more detailed analysis is required. This paper presents a computational method to handle such cases, particularly where non-linearity appears in the diffusion problem. The method involves both orthogonal collocation and matrix transformations to compute approximate concentration profiles and impedance behavior. An example of the method is given for the case of a catalytic EC mechanism, where a homogeneous reaction in the diffusion layer is coupled with the electrode process. The analysis shows how the reaction decreases the impedance of the diffusion layer by regenerating the electroactive reactant. (C) 2010 Elsevier Ltd. All rights reserved.