In contrast to what is observed in alkaline media, cyanide adlayers formed on Pt(100) are very reactive in 0.1 M HClO4 solutions. In this medium, dissimilar behaviour has been observed for the potential regions above and below 0.4 V (RHE). CNads is oxidised above 0.8 V to form dissolved CO2 and an adsorbed nitric oxide layer. Two distinct absorption frequencies have been observed for the N-O stretching at 1610 and 1680 cm(-1). On the other hand, CNads is able to react below 0.4 V to yield an adsorbed carbon monoxide adlayer. The reaction rate is slow enough to allow the observation of a surface intermediate species by means of in situ FT IR spectroscopy. As deduced from the analysis of the spectra, this species may contain a double C-N bond (i.e. an iminium-like adsorbed residue).