The surface species formed following acetylene adsorption on Pd(lll) at 300 K is identified as a vinylidene. It is oriented with its carbon-carbon axis tilted at similar to 50 degrees to the surface normal which allows overlap between molecular pi orbitals and the surface d-orbitals. The carbon-carbon bond is slightly longer (by similar to 0.03 Angstrom) than for adsorbed ethylene on Pd(111). This assignment is consistent with previous photoelectron and high-resolution electron energy loss spectroscopy data. Adsorbed vinylidene decomposes to yield hydrogen and deposit carbon onto the surface at similar to 480 K with an activation energy of similar to 115 kJ/mol.