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The kinetics of catalyzed hydrogenation of ethylene in the presence of nickel wires was studied over a range of partial pressures both of ethylene and hydrogen from about 0.017 to 0.15 mmHg, taking special precautions against the decay of catalytic activity. The catalysts were activated by oxydation followed by reduction and annealing before every series of runs of the catalyzed hydrogenation. Several series of the catalyzed hydrogenation runs were conducted by evacuating and reducing the catalyst after each run at different hydrogen and ethylene partial pressures. The initial rate of hydrogenation of each run was thus found proportional to the 0.7th power of hydrogen partial pressure pH but independent of ethylene partial pressure pE, i.e. proportional to (PH)O.7(PE)O as investigated at DoC and-45°C. The activation energy was determined to be 5.3 ± 0.3 Kcal/mole from the initial rate observed separately at-22°C, DoC, lloC and 25.5°C. Series of another sort were carried out by keeping the catalyst in ethylene vapour (0.03 mmHg) saturated at liquid oxygen temperature and starting every run just by supplementing hydrogen consumed without evacuation or any other preceding treatment of catalyst. The decay of catalytic activity with each run of a series was less than 5;10 and the rate of hydrogenation was proportional to (PH)O.7 at O°C, 25.5°C and 40°C. The optimum temperature was thus found about 60°C and the activation energy below the optimum 5.3 ± 0.2 Kcal/mole in coincidence with in case of the series of the first sort. The decay of catalytic activity with each run amounted, however, to 20;10 or more above the optimum, excluding any quantitative determination of activation energy there