The adsorption of cyanide, CN, on the Pt(111) surface by dissociation of acetonitrile, CH3CN, has been investigated by sum-frequency generation (SFG) as surface vibrational spectroscopy and by cyclic voltammetry. The adsorption procedure involved cooling of the flame-annealed Pt crystal above a dilute aqueous solution of acetonitrile (0.1 M HClO4 + 0.025 M CH3CN), followed by immersion of the sample into the electrolyte. The adsorbed species is identified as cyanide by the characteristic potential dependence of the stretching frequency, which increased from 2087 cm(-1) at 0.1 V to 2135 cm(-1) at 0.95 V, as well as by cyclic voltammetry. Comparison with the voltammogram in a 1 x 10(-3) M solution of CH3CN indicates that the dissociation takes place in the gas phase above the solution. A second band in our spectra with a frequency near 2155 cm(-1) is tentatively assigned to a Pt-cyano surface complex. We also report on experiments in which the same annealing/cooling procedure has been applied using neat liquid acetonitrile. Bands near 1860 and 2073 cm(-1) in SFG spectra of these samples are shown to originate from adsorbed CO, most likely produced by oxidation of CH3CN vapor at the hot Pt crystal. CN adsorption at the Pt(111)/liquid acetonitrile interface, performed by deposition of cyanide ions from solutions of tetrabutylammonium cyanide in acetonitrile, is characterized by a single band near 2110 cm(-1).