The features of digital simulation, analytical modelling and semianalytical modelling are contrasted. Two of these approaches are applied to an electrolysis experiment in which a neutral substrate, in solution in a thin-layer cell, is oxidized to an ionic product. The resistance of the cell, initially high, diminishes with time in the constant-current experiment. The cell voltage reflects both the decreasing ohmic polarization and the increasing concentration polarizations. Ultimately, exhaustion of substrate at the anode leads to a catastrophe at a transition time. The experiment is modelled simulatively and analytically and the results are compared. A bookkeeping procedure is employed to assess the reliability of the numerical model. (C) 2004 Elsevier B.V. All rights reserved.