Oligomerization could effectively upgrade the n-butenes in Raffinate II to more valuable octenes. In order to use this feedstock, however, one must consider the effects of butadiene and isobutylene on the catalyst. Experiments were conducted with industrially supplied Raffinate II, and with pure l-butene, to study the influence of these compounds and reaction conditions on the activity of NiY zeolite catalyst. Results show that isobutylene increases the activity of the catalyst, but that butadiene strongly deactivates the catalyst even at low levels. Evidence suggests that deactivation is caused by butadiene irreversibly complexing with active nickel sites, and by pore blockage. We propose that the oligomers blocking the zeolitic pores are formed via the Diels-Alder reaction. Catalyst deactivation is so severe that Raffinate II cannot be used with Ni catalysts unless steps are taken to remove all traces of butadiene from the feed.