Cyanide-modified Pt(1 1 1) surfaces have been recently used to study atomic-ensemble effects in electrocatalysis. These studies have been based on the assumption that cyanide acts as a third body, blocking some surface sites but leaving the electronic properties of the surrounding ones unaltered, although this is in apparent contradiction with the observation of a positive shift of 0.2 V in the onset of hydrogen adsorption on cyanide-modified Pt(1 1 1) electrodes, as compared with clean Pt(1 1 1). We have performed theoretical calculations in order to provide support to this assumption and explain the positive shift in the onset of hydrogen adsorption, which is shown to be due to the formation of CNH_ad.