The kinetics of ethylene hydrogenation on iridium (111) single crystals were measured in the temperature range of 312K to 625K. The pressure of hydrogen was varied from.4 to 626 torr and the pressure of ethylene was varied from.08 to 588 torr to determine the kinetic order of each reactant at 473K. The hydrogen kinetic order was determined to be 1. The ethylene kinetic order varied from 1 at low pressures to 0 at higher pressures. The data were fit to a Langmuir Hinshelwood mechanism;The temperature dependence of ethylene hydrogenation yielded an activation energy of 12.5 Kcal/mole between 312K and 400K. A maximum in the temperature versus rate plot is attributed to a change in the form of the adsorbed ethylene which is hydrogenated on the surface. Thermal desorption and Auger electron spectroscopies supported this argument by providing additional information on the nature of partially hydrogenated carbonaceous surface species on the surface after ethylene hydrogenation;The yield of side product, methane, was determined under the same conditions as the hydrogenation reaction. Methane was observed to originate from ethylene and not hydrogenolysis of the hydrogenation product, ethane. The quality of the data did not allow accurate determination of the orders for methane formation. The activation energy is 4.88 Kcal/mole. ftn * DOE Report IS-T 1248. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.