The room temperature desorption and exchange of CO in a saturated CO adlayer on a Pt electrode, at potentials far below the onset of oxidation, was investigated by isotope labeling experiments, using a novel spectroelectrochemical setup, which allows the simultaneous detection of adsorbed species by in situ IR spectroscopy and of volatile (side) products and reactants by online mass spectrometry under controlled electrolyte flow conditions. Time-resolved IR spectra show a rapid, statistical exchange of pre-adsorbed (13)CO(ad) by (12)CO(ad) in (12)CO containing electrolyte; mass spectrometric data reveal first-order exchange kinetics, with the rate increasing with CO partial pressure. The increasing CO(ad) desorption rate in equilibrium with a CO containing electrolyte is explained by a combination of an increasing CO(ad) coverage upon increasing the CO pressure, and a decrease of the CO adsorption energy with coverage, due to repulsive CO(ad)-CO(ad) interactions.