The catalytic activity and selectivity for ethylene dimerization have been investigated on Ni(II)-exchanged clinoptilolite using a static reactor and gas chromatography. The catalytic results indicate that ethylene is dimerized to n-butenes at high temperature via direct reduction of Ni(II) by ethylene in Ni-clinoptilolite containing different cocations. This is consistent with electron spin resonance (ESR) results observed after adsorption of ethylene on Ni(II)-containing clinoptilolite at various reaction temperatures. Our ESR results suggest that Ni(II) in the clinoptilolite structure is reduced by direct interaction with ethylene, leading to the formation of stable Ni(I)-(C2D4)(n) complexes. At a later stage, several ESR species assigned to Ni(I) complexes with butene are produced as a result of ethylene dimerization. The catalytic performance of Ni-clinoptilolite in ethylene dimerization is dependent on the reaction temperature, the type of cocation and the amount of nickel ions incorporated into extraframework sites of clinoptilolite. Along with n-butenes as major products at the initial stage, side products such as methanol, butane, and isobutene increase with reaction time at 623 K. This leads to a decrease in the selectivity for n-butene at longer reaction time. The selectivity for n-butene is higher at a lower reaction temperature of 523 K, compared to that observed at 623 K, whereas the catalytic activity described as the maximum percent of ethylene converted to n-butene is higher at 623 K. This is explained by a higher ESR intensity of Ni(I) species formed at 623 K. Our catalytic results also show similar catalytic activity no matter what type of cocation is present. This is expected by the ESR results showing no large difference in the reducibility of Ni(II) in activated samples containing different cocations. However, the replacement of a smaller cocation with a larger one like Cs+ increases the selectivity for n-butenes. More NI(II) incorporated into extraframework sites of clinoptilolite increases the formation of n-butenes but also causes deactivation more quickly due to reduction of active Ni(I) to inactive Ni(0) by ethylene and butene.