The effect of the addition of structure-modifying agents such as ZnO on the processes of coke formation and sulphur poisoning of NiO/Al2O3 catalyst during ethyne hydrogenation has been studied. With or without the presence of modifiers, the addition of H2S to the feed gives rise to a strong increment in the rate of catalyst coking. However, pretreating the catalyst with H2S has a different effect when ZnO is added to the catalyst. In a NiO/A1(2)0(3) catalyst, sulphur preferentially deactivates metallic sites which are very active for coke formation. However, when ZnO is present in the catalyst structure, the addition of sulphur gives rise to a lower hydrogenation yield, and a greater rate of coke formation with respect to the fresh catalyst. Pretreating the catalyst with H2S also changes the coke morphology.