The peculiarities of an amperometric biosensor based on parallel substrates conversion are investigated digitally. The mathematical model of the biosensor is based on a system of non-linear reaction-diffusion equations. The model involving the enzyme and diffusion layers is defined in a one-dimensional domain. By changing input parameters the biosensor action is analysed numerically with a special emphasis to the influence of the catalytic activity and the geometry of the biosensor on its response and sensitivity. The digital simulation at the transition and steady-state conditions was carried out by using the finite difference technique. Calculations showed that the half maximal effective concentration of the substrate varies by several orders of magnitude by changing the concentrations of enzymes, the hydrogen peroxide concentration as well as the enzyme layer thickness. The behaviour of the biosensor sensitivity and the half maximal effective concentration is especially complex at large concentrations of the catalase as well as low concentrations of the peroxidase. (C) 2012 Elsevier B.V. All rights reserved.