The reaction of CO with (NO)-N-15 and (NO)-N-14 mixtures in a co-adsorption layer on the Pt(100)-(hex) surface was studied by TPR. The kinetic isotope effect (KIE) manifests itself in the variation of the temperature of the maximum of the N-2 desorption peak depending on the isotopic composition : T-max(N-14(2)) < T-max((NN)-N-14-N-15) approximate to T-max(N-15(2)). The KIE observed is consistent with the assumption that the NOads dissociation is the rate-determining step of the reaction.