The oxidation of CO on Ir(111) surfaces was investigated under UHV conditions in the temperature range 360 K to 700 K by CO2 rate measurements utilizing mass spectroscopy. Steady-state CO2 rates were measured at constant total CO+O-2 gas flux and variable gas composition (Y-CO=Y, Y-O2=1-Y) using mass flow controllers which allowed changes in the CO/O-2 gas composition down to 0.1%. Between 360 K and 450 K the CO2 rates initially increase proportional to Y (T<400 K) or to Y-1.5 (420 K