The non-steady energy and mass balances for a catalytically coated channel-passed by a reacting gas flow yield a stiff nonlinear system of differential and algebraic equations, which is very time-consuming to solve. For reactions following Langmuir-Hinshelwood-kinetics, e.g. the catalytic oxidation of CO over Pt, an analytical solution of the wall mass balance is possible. Assuming quasi-static behaviour of the gas phase and solving the gas phase balances by Runge-Kutta routines leads to an efficient, reliable code.