The theoretical analysis of the steady-state amperometric response for conducting polymer-modified ultramicroelectrodes is discussed. The effect of substrate diffusion in the solution adjacent to the polymer film on both the concentration profile and current response is also examined. Simple analytical expressions for substrate and mediator concentrations and current responses for all values of reaction/diffusion parameters are presented. The model is based on non-stationary system of coupled reaction/diffusion equations containing a non-linear term related to Michaelis-Menten kinetics of the enzymatic reactions. He's variational iteration method is used to give approximate analytical solutions of coupled non-linear reaction diffusion equations. A good agreement with available limiting case results is noticed. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.