Using an atom superposition and electron delocalization molecular orbital (ASED-MO) method, we have investigated the vibrational and chemisorptive properties of adsorbates on a Pt(100) surface during CO oxidation. The calculated vibrational stretching frequency for a predicted structure of $[CO{cdot}{cdot}{cdot}O]^*$ complex is 1642 $cm^{-1}$. The CO bond stretches by 0.05 ${AA}$ when adsorbed on one-fold site, and is tilted by 30 ${AA}$ from the surface normal. We find the decrease in CO vibrational frequency on going from the one-fold to the high coordination sites. Binding at the two-fold site is predicted to be favored for $Pt_{18}(100)$ and at the 1-fold site for $Pt_{23}(100)$. From the calculations of the steric interactions, we have found that pre-adsorbed oxygen modifies the surface so that CO is adsorbed on the one-fold site ordered in a $(sqrt{2}{ imes}{sqrt}{2})R45^{circ}$. Our results are in good agreement with recent experimental findings of Hong et al. [J.Phys. Chem. 1993, 97, 1258].