Reflection absorption infrared spectroscopy and temperature-programmed desorption were used to investigate the adsorption of methyl isocyanide (CNCH3) on the Pt(111) surface and its reaction to form methylaminocarbyne (CNHCH3). In agreement with previous studies, methyl isocyanide is found to adsorb at on-top sites at low coverages and at both on-top and 2-fold bridge sites at higher coverages. Hydrogen reacts with the N-atom of methyl isocyanide adsorbed at on-top sites to form methylaminocarbyne, whereas at higher coverages where bridge-site occupation occurs the reaction is suppressed. The formation of an NH bond is indicated by the appearance of a sharp peak at 3405 cm(-1) upon annealing a low coverage of methyl isocyanide to temperatures of 200 to 280 K. The formation of the NH bond is accompanied by loss of the intense isocyanide stretch at 2235 cm(-1) and its replacement by a new set of peaks in the 1200-1500 cm(-1) region that are readily assigned to the modes of methylaminocarbyne. Spectral assignments and interpretation are aided by results for the CNDCH3 and (CNHCH3)-C-13; isotopomers.