Experiments on the catalytic oxidation of carbon monoxide (CO) in a wall-cooled monolithic catalyst carrier are presented. The reactants, air with up to 2 mol.% of carbon monoxide, streamed through the square cross-sectional channels of the monolith and were converted partially to carbon dioxide (CO2); see Fig. 1. The dependence of the solid phase temperatures and the conversion of CO on the residence time of the reactants in the catalyst as well as the dependence of conversion on the wall temperature and the reactants’ CO content are shown experimentally. Two models dealing with heat transfer in monoliths were extended by a reaction term to describe the communicated experiments. A comparison of calculated and measured temperatures and conversions is given.