We have observed, using infrared spectroscopy, that the precursor-mediated O2 chemisorption on the clean and the partially hydrogen-covered Pt(111) surfaces exhibits opposite temperature dependencies above the temperature for stable O2 physisorption. While the chemisorption probability on the clean surface increases with increasing temperature due to thermal activation of the precursor, it decreases on the partially hydrogen-covered surface which we suggest is due to a general loss of the mobile precursor molecules by thermal desorption from chemically inert hydrogen islands.