Nickel catalysts tend to deactivate in the hydrogenation of acetonitrile. Using temperatureprogrammed techniques, combined with a number of different analytical procedures, including mass spectrometry, it was established that the formation of surface nickel carbides was the major cause of the loss of activity with time on stream. The presence of partly dehydrogenated species was a second factor causing the loss of activity. Even in the presence of hydrogen adsorbed nitrile molecules were partly dehydrogenated during the process. The resulting species were strongly adsorbed on the nickel surface and blocked some of the active sites for hydrogenation. Treatment of the deactivated catalyst with hydrogen at an elevated temperature (\left(>200^{\circ} \mathrm{C}\right)) restored both the catalytic activity and the selectivity of the catalyst. The influence of the hydrogen partial pressure on the rate of deactivation was assessed by performing the hydrogenation at several different molar hydrogen-to-acetonitrile ((\mathrm{H} / \mathrm{C})) ratios. It was found that the performance of the catalyst could be maintained when a large excess of hydrogen was present in the feed. Under the experimental conditions, a (\mathrm{H} / \mathrm{C}) ratio higher than about 25 was required to maintain productivity. 1993 Academic Press, Inc.