The effect of NO on the heterogeneous catalytic surface reaction CO-O/sub 2/ and the effect of O/sub 2/ on the CO-NO reaction in studied by the Monte Carlo simulation technique. The overall kinetic behavior of the three component reaction is seen to be dominated by the characteristics of the CO-NO reaction rather than by the CO-O/sub 2/ reaction. The addition of NO at the expense of O/sub 2/ in the CO-O/sub 2/ reaction leads to an enhanced production of both CO/sub 2/ and N/sub 2/. For the NO-CO reaction the addition of O/sub 2/ at the expense of NO decreases the production rates of these two gases. The experimentally observed blocking effect of NO on the CO-O/sub 2/ reaction is not observed in the simulation. For the blocking effect to be realised the un-dissociated NO on the surface plays a crucial role. Our simplified model assumes complete dissociation of NO, consequently the blocking effect is not observed. The effect of the third component (NO or O/sub 2/) on the `critical transition` leading to poisoning of the two component reaction is also studied. 7 figs. (author).