Using infrared reflection absorption spectroscopy we have investigated how preadsorbed hydrogen affects the adsorption of O-2 on the Pt(111) surface at temperatures below the onset of the water formation reaction. On the fully hydrogen covered surface, Theta(H)=1, O-2 physisorbs at temperatures below 45 K, the weakly dipole active internal stretch vibration is observed at 1548 cm(-1). Unlike on the clean Pt(111) surface, this adsorption state does not act as a precursor for O-2 chemisorption. The physisorbed molecules simply desorb above 45 K and no chemisorbed O-2 state is populated directly from the gas phase in the temperature range 45-90 K. When the surface is approximately half covered, Theta(H)approximate to0.4, with preadsorbed hydrogen, O-2 chemisorbs on the clean Pt(111) surface regions in the characteristic peroxolike and superoxolike states with vibration frequencies around 700 cm(-1) and 870 cm(-1). These values correspond to dense O-2 islands which develop already at low O-2 coverages. At this hydrogen coverage, we find that the initial sticking probability of chemisorbed O-2 is drastically reduced at 90 K and the general uptake also proceeds slowly when compared with observations for the clean surface. We suggest that this is due to a change in the behavior of the physisorbed O-2 precursor. (C) 2004 American Institute of Physics.