The photoactivation of chemisorbed O-2 in the presence of chemisorbed CO on Pt(111) has been investigated for uv light in the range 3.87-4.77 eV (260-320 nm). Three photoprocesses first-order in O-2 coverage have been separated and for the first time the cross sections for each are reported. The dominant process is O-2 photodissociation (Q(diss) = 4.0 +/- 0.1 X 10(-21) cm(2)). The second most probable process is photodesorption (Q(des) = 2.2 +/- 0.1 X 10(-21) cm(2)). The least probable process is photoreaction with chemisorbed CO (Q(rxn) = 0.35 +/- 0.03 X 10(-21) cm(2)). Previous studies of Q(rxn) have reported cross sections as high as 5 X 10(-17) cm(2).