We have used the CO charge-displacement method, in combination with a thermodynamic cycle, to obtain the double-layer correction necessary to determine accurately, using the charge in the corresponding CO-stripping voltammograms, the maximum amount of CO that can adsorb on a cyanide-modified Pt(111) electrode. The resulting CO coverage at saturation is theta(co) = 0.25, and corresponds to a mixed CN-CO adlayer where some Pt atoms are still free and consequently can adsorb hydrogen. The hydrogen adsorption charge for the mixed adlayer, obtained from the corresponding cyclic voltammogram, agrees very well with that estimated from the CO and CN coverages, assuming that one hydrogen atom adsorbs on every free Pt atom. Taking into account these data, we propose a structural model for the mixed CN-CO adlayer on Pt(111). (C) 2003 Elsevier B.V. All rights reserved.