The coadsorption of CO and Cs on Pt{111} at 100 K has been investigated using infrared reflection-absorption spectroscopy (IRAS) and low energy electron diffraction (LEED). The individual coverages of both adsorbates were varied over a wide range and the order of adsorption was reversed. The adsorption behavior is similar to that of CO coadsorbed with K on Pt{111}. The vibrational data show four groups of strongly perturbed C-O stretches with frequencies between 1420 and 1810 cm-1 as well as several less perturbed bands between 2000 and 2100 cm-1. Only two ordered coadsorption structures are observed : a (4X2) and a (2X2), the latter existing over a wide Cs coverage range, indicative of CO-Cs island formation. As in the case of Pt{111}-CO/K, the IRAS results are interpreted in terms of a local stoichiometry picture. The differences between the two coadsorption systems appear to be related to the different size of the alkali-metal atoms rather than to electronic effects.