Electrocatalysis of CO oxidation and the interfacial structure of the CO adlayer (COad) on the Pt(111) surface in 0.5 M H2SO4 were examined by using the rotating disk electrode method in combination with in situ surface X-ray diffraction measurements. The results presented here elucidate the roles played by two different forms of COad: one which is oxidized at lower overpotentials, in the so-called preoxidation region, we characterize as a weakly adsorbed state (COad.w), and a strongly adsorbed state (COad,s) which is oxidized at higher overpotentials. The COad,w state forms at saturation coverage by adsorption at E < 0.15 V and assumes a compressed p(2 x 2) structure containing 3 CO molecules in the unit cell (0.75 CO/Pt). We propose that oxidative removal of COad,w is accompanied by simultaneous relaxation of the CO adlayer, and that the remaining COad (approximate to 0.6 CO/Pt) assumes a new bonding state which we identify as COad,s. The COad,s state is present in a structure lacking long-range order. Despite the reduced coverage by COad, H-2 electrooxidation is still completely poisoned at potentials below 0.6 V. The electrooxidation of CO in solution is proposed to occur via the COad,w state at 0.6-0.8 V, and via the COad,s at higher potentials.