Coadsorption of chlorine and carbon monoxide on a Pt(100) face is studied in an ultrahigh vacuum system using line-of-sight desorption mass spectrometry and LEED. Adsorption of the single components as well as coadsorption of both species removes the reconstruction of the clean Pt(100) hex surface. On a partially chlorine-precovered Pt(100) (1x1) surface, adsorption of CO occurs with high efficiency via a precursor state up to the completion of a mixed CO/Cl c(2x2) adlayer. The rate of the slow CO uptake that occurs further is determined by the transformation of the adlayer into a densely packed arrangement with a total adsorbate-to-surface Pt atom ratio of about 0.6. Depending on the dosing conditions, the mixed high-coverage adlayer exhibits the LEED patterns of a c(2x2) or of a ((50)(11)) structure. Temperature-programmed desorption produces separate carbon monoxide and Cl atom peaks. The desorption kinetics indicate that both electronegative adsorbates, CO and Cl, destabilize CO in the c(2x2) adlayer to a comparable extent.