The pathways for the hydrogenation of adsorbed acetylene and vinylidene on Pd(lll) are investigated using temperature-programmed desorption and infrared reflection-absorption spectroscopies. The chemistry of the vinyl intermediate formed by the hydrogenation of both species is investigated by adsorbing vinyl iodide on Pd(lll) where it is found that vinyl species hydrogenate more rapidly than adsorbed acetylene, indicating that the rate-limiting step in acetylene hydrogenation is the addition of the first hydrogen to acetylene to form a vinyl species, Infrared spectroscopy also reveals that vinyl species convert to ethylidynes as Pd(lll) is heated above similar to 160 K. The hydrogenation of vinylidene by up to similar to 0.2 Torr of hydrogen involves intermediate ethylidyne species in accord with this observation. Surprisingly, the rate constant for the conversion of vinylidene into ethylidyne is identical to that for the titration of ethylidyne from the surface by hydrogen, an effect that may be explained in terms of the different saturation coverages of the two species.