Interaction of ethylene with the H-preadsorbed Pd(110) surface has been studied by the use of thermal desorption spectroscopy and high-resolution electron energy loss spectroscopy. On the Pd(110)(2 × 1)-H surface saturated with ethylene at 90 K, weakly-adsorbed ethylene exists in addition to the π-bonded ethylene. The fractional saturation coverage of ethylene is 0.58, which is nearly equal to that on the Pd(110) clean surface. The amount of ethane which is thermally desorbed is about 15 times as large as that from the Pd(110) clean surface. The ethane formation is promoted as the amount of preadsorbed hydrogen is increased. The H-D-exchange reaction for C2D4 postadsorption on the H-preadsorbed Pd(110) surface is discussed. Experimental evidences are given which indicate that the ethane formation and H-D-exchange reaction occur predominantly via ethyl species. The interaction of hydrogen with the ethylene-preadsorbed Pd(110) surface is briefly described.