The reaction kinetics of ethylene oligomerization was investigated over the temperature range 278-323 K and near-atmospheric pressure in a laboratory slurry reactor. The nickel/sulfated alumina catalyst was prepared by use of a non-porous fumed alumina (ALONT(TM)) support and by ion-exchange thereon of Ni ions. A first-order dependence of ethylene consumption rate on ethylene concentration in iz-heptane solvent was determined with an apparent activation energy of 16.3 kJ/mol. Several size fractions of catalyst particles were also used to study the external mass transfer resistance and the internal resistance to pore diffusion and chemical reaction. The results indicate that neither of the resistances can be ignored. The combined internal diffusion and reaction steps appear to be rate controlling for ethylene oligomerization in the slurry reactor. The influence of gas absorption resistance was also evaluated by carrying out experiments using different charges of catalyst mass. The selectivity to 1-C-6 increases with temperature. At temperatures < 298 K, the dominant products are l-butene and l-hexene with traces of their isomers.