The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO . ZnO . Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.