The surface chemistry of HCN on Pt(111) and Cu(100) is markedly different. On Pt( 111), temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) show a series chemical transformations with increasing temperature, whereas on Cu(100), HCN desorbs below 170 K without dissociation. Despite these differences, adsorption of HCN on Cu(100) and Pt(lll) at 90 K and below yields very similar infrared (IR) spectra. The IR spectra are characterized by an intense CH stretch fundamental at 3294 cm(-1) on Cu(100) and at 3298 cm(-1) on Pt(lll). The closeness of these peaks to the values for HCN in the gas phase or isolated in argon matrices as well as the weakness or absence of other fundamentals indicate that the molecule bonds weakly to both surfaces via the nitrogen lone pair with the molecular axis oriented along the surface normal.