The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identified by high-resolution electron energy loss spectroscopy to be ethylidyne (C−CH₃). This study is the first direct spectroscopic observation of a C−CH₃ species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C−CH₃, ¹³C−¹³CH₃, C−CD₃, and ¹³C−¹³CD₃, are reported, and a complete and consistent vibrational assignment of their fundamental modes is presented. Based on this assignment, a force field is derived from the measured vibrational frequencies using a normal-modes analysis and is found to be in good agreement with that deduced from IR spectra of an ethylidyne species in an organometallic complex. Inspection of the eigenvectors of the normal-mode displacements reveals that substantial mixing of harmonic bond motions is the origin of the unusual upshift in frequency of the C−C stretching mode upon deuteration. A quantitative determination of the relative dynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C−C stretching and symmetric CH₃ deformation modes upon deuteration, phenomena common to all C−CD₃ spectra, also arise from extensive mixing of bond motions. A detailed analysis of the spectra strongly suggests a C_3v or C₃ local environment for ethylidyne and a 3-fold hollow adsorption site.