Oxidation of CO on IR(111) was investigated between 420 and 530 K under a constant CO + O-2 gas flux and varied fractions of CO (Y) and O-2 (1 - Y). Upon variation of Y in the 'up' and 'down' direction, the stationary CO2 rates exhibit bistability below 530 K between two temperature dependent limiting Y values. An analysis of the bistability at 500 K revealed that the equilibrium CO2 rates on the two branches are stable in excess of 20 h. The observed phenomena, including feed gas pulsing effects, are reproduced by the solutions of appropriate kinetics equations. (C) 2003 Elsevier Science B.V. All rights reserved.