The kinetics of the hydrogenation of ethylene on Pt(111) was studied isothermally and under vacuum by using a variation of the dynamic molecular beam method originally devised by King and Wells. At surface temperatures above 240 K ethylidyne formation competes with both ethylene hydrogenation and ethylene desorption. At temperatures below 240 K, on the other hand, the decomposition of ethylene is slow, and the adsorption and hydrogenation kinetics for ethylene on both clean and hydrogen-covered surfaces could be investigated independently. Ethylene adsorption was found to be precursor-mediated at low coverages and Langmuirian near saturation. A certain population of weakly-adsorbed species can also be maintained at coverages near saturation by exposure of the surface to a constant flux of ethylene molecules. The presence of coadsorbed hydrogen reduces the total ethylene uptake but increases the amount of weakly-adsorbed ethylene as compared to that on the clean Pt(111). The main conclusion from this work is the fact that this weakly-adsorbed species appears to be essential for the hydrogenation of ethylene : the kinetic orders of the reaction were determined to be 1.2 +/- 0.3 and 0.8 +/- 0.2 with respect to the weakly-adsorbed ethylene and hydrogen surface coverages, respectively. An activation energy of 6 +/- 1 kcal/mol was measured for the hydrogenation of ethylene to ethane under the conditions of these experiments. Finally, the presence of ethylidyne on the surface was found to not influence the hydrogenation reaction in any other way than by blocking surface sites.