The hydrogenation of ethylene on Pd(111) is probed using a combination of temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Ethylene adsorbs on clean Pd(111) in a di-sigma configuration but converts to pi -bonded species when the surface is presaturated by hydrogen. Ethane is formed with an activation energy of 3.0 +/- 0.3 kcal/mol only when Pd(111) is pre-covered by hydrogen and not when ethylene and hydrogen are co-dosed, indicating that ethylene blocks hydrogen adsorption. Experiments performed by grafting ethyl species onto the surface by reaction with ethyl iodide indicate that ethyl species hydrogenate much more rapidly than the overall rate of ethylene hydrogenation, demonstrating that the addition of the first hydrogen atom to adsorbed ethylene to form an ethyl species is the rate-limiting step in the hydrogenation reaction. The adsorption geometry of ethyl iodide is found to depend on dosing conditions. When adsorbed at low exposures at 80 K, the mirror symmetry plane of ethyl iodide is oriented close to parallel to the surface. At higher exposures, it adopts a geometry in which the symmetry plane is closer to perpendicular to the surface.