The interactions of satd. layers of molecularly adsorbed O₂ on Pd(111) with post-dosed CO have been studied with temp. programmed desorption (TPD) and electron energy loss spectroscopy (EELS) in the surface temp. range 100-230 K. It has been found that CO does not react chem. with O₂(a) to form CO₂. CO does react with adsorbed O atoms at higher surfaces temps. resulting in the prodn. of CO2. Introducing CO to a satd. O₂ layer decreases the total O₂(a) coverage as well as changing the relative populations of the three chemisorbed O₂ species.