Ethylene-C 14 tracer studies have been made of the adsorption and hydrogenation of ethylene at 20°C using 5% alumina-supported nickel, rhodium, palladium, iridium, and platinum catalysts. The effects of an equimolar mixture of nonradioactive ethylene and hydrogen, nonradioactive ethylene, and hydrogen on preadsorbed ethylene-C 14 have been observed using an end-window G-M counter placed within the catalyst vessel. In all cases only part of the preadsorbed ethylene was removed from the surface and it was independent of the subsequent treatment. Average percentages of initial radioactivity retained in the surface were Pd, 63.5%; Ni, 24.0%; Rh, 22.5%; Ir, 16;% and Pt, 6.5%. Evacuation following ethylene-C 14 adsorption gave similar retention values. With nickel catalysts, pretreatment with hydrogen at 20°C did not change the retention of radioactivity, and at surface saturation an equilibrium between gas-phase and adsorbed ethylene was observed. Ethylene-C 14 containing 8% acetylene-C 14 behaved similarly to ethylene-C 14 with respect to retention of radioactivity. However, acetylene-C 14 could displace ethylene from the metal surfaces. Attempts to elucidate the nature of the adsorbed species have been made, correlations being sought between retention, heat of adsorption, and the ability of the metals to break carbon-hydrogen bonds. A possible correlation between retention and selectivity for acetylene hydrogenation is also discussed.