A cylindrical mirror electron energy analzyer has been used to study the adsorption and decomposition of CO on an initially clean Pt(111) surface. The observed rate of adsorption and fractional surface coverage as a function of CO exposure are identical to those observed previously in this laboratory by flash filament adsorption technique. This result provides an absolute calibration of the Auger spectrometry system for carbon and oxygen. Beam-induced decomposition of CO was observed at high incident fluxes. The surface oxygen coverage decreased exponentially to zero during electron bombardment, while surface carbon decreased and then levelled off at a finite value. CO adsorption studies on this carbon contaminated surface showed decreasing CO adsorption rate and decreasing saturation coverage with increasing carbon contamination. The adsorption rate approached zero at high carbon concentration.