A theoretical analysis describing the transient current response of a conducting-polymer-based amperometric chemical sensor to a concentration step is presented. The processes of substrate transport and substrate reaction within the polymer matrix are examined by solution of the time-dependent diffusion-reaction equation. Analytical expressions for the concentration profiles in the layer and the amperometric sensor response are presented. The shape of the response is shown to be in good agreement with that obtained experimentally for the amperometric detection of ascorbate at polypyrrole electrodes.