Vibrational properties of CO have been studied on Pt(111) in acid and alkaline electrolytes by synchronous measurements of CO oxidation current (0.5 mV/s) and WAS spectra (one spectrum for every 1 mV). We found that in acid solutions the frequency-tuning rate (dv(CO)/dE) as well as the potential-dependent bandwidth (dDeltav(1/2)/dE) deviates from expected linear relationships. This unusual potential-dependent behavior is interpreted in terms of compression/dissipation of CO islands during the CO oxidation, engendered by competitive adsorption between inactive anions from a supporting electrolyte and the reactive OH species.