Nickel exchanged Y zeolite (NiNaY) catalyzes ethylene dimerization at 50-70-degrees-C, 100-600 psig (1 psig = 6.895 kPa) and an hourly space velocity of 2 h-1 to give primarily 1-butene together with cis- and trans-2-butenes. Nickel is required for activity since an acidic HNaY zeolite is inactive for ethylene dimerization. To distinguish between the role of nickel and acid sites (Lewis and Bronsted) of the zeolite on the dimerization of ethylene and isomerization of 1-butene, the activity of NiNaY modified with different acids and bases was investigated. The acidity of NiNaY and modified NiNaY were determined by temperature-programmed desorption of ammonia. Treatment of NiNaY with pyridine and ammonia which selectively poison the Bronsted acid sites results in reduced activity and no improvement in selectivity to 1-butene. Calcination of NiNaY at 700-degrees-C which converts most of the Bronsted acid sites to Lewis acid sites also reduces the activity significantly. Modification of NiNaY with picolinic acid reduces the activity due to chelation of nickel by the anion of picolinic acid and increases the isomerization activity due to the release of H+ onto the zeolite. These results suggest that both the nickel and Bronsted acid sites are required for dimerization of ethylene. Dimerization of ethylene and isomerization of 1-butene are intrinsically related on the NiNaY catalyst. Ni+ - H+ is proposed to be the active site. Modification of NiNaY with picolinic acid apparently provides an additional pathway for isomerization of 1-butene.