Abstract The ohmic and capacitive components of the impedance of a smooth Pt wire electrode were measured at a cathodic current density of −1 mA cm −2 at frequencies ν between 10,000 and 25 Hz at different coverages of chemisorbed carbon monoxide in 0.5 M H 2SO 4. The linear dependence upon ω −1 2 , characteristic for predominant H 2 diffusion on clean Pt electrodes, disappears at carbon monoxide coverages above 0.2. The hindrance of the H 2 evolution reaction increases with the coverage of CO ad in a gradual fashion below 0.7 and rapidly at larger coverages. The hindrance is considerably larger on the Pt wire than on Pt foils, investigated previously. The change of the attenuation factor, caused by the blocking of the free surface, cannot account for the observed effect on the Pt wire electrode. Simple models, based on the Volmer—Tafel or Volmer—Heyrovsky mechanisms for H 2 evolution, cannot describe the experimental frequency dependence of the impedance at coverages above 0.2.