Recent trends in surface chemistry are trying to bridge the gap between ideal ultrahigh vacuum (UHV) studies and results of catalytic reactions under industrial conditions. These conditions are mainly characterized by a heterogeneous active phase, high pressure and the presence of contaminant species. In the present work we address the last of these three factors. More precisely we study the effects of a contaminating species, whose only action is adsorption and desorption being otherwise inert, on the kinetics of CO oxidation on transition metals. We introduce a suitably modified Ziff-Gulari-Barshad model, a contaminated ZGB model, and obtain by Monte Carlo simulations the behavior of the steady state of the system for different CO and contaminant concentrations. We find that the reaction window shrinks with increasing contaminant concentration until it disappears at a critical value. Moreover, we find that the classical lower bound ZGB irreversible phase transition is unaffected while the upper bound first order transition transforms continuously into a second order one with increasing contaminant concentration, as revealed by the behavior of critical exponents.