Nickel-containing mesoporous materials MCM-41 and AlMCM-41 have been prepared by synthesis and ion exchange. These materials are catalytically active for ethylene dimerization and 1-butene isomerization. The catalytic activity for the dimerization reaction is shown to be due to nickel(I) species, while the 1-butene dimerization is dominated by the acidic properties of the MCM-41 materials. The ethylene dimerization activity and product distribution are different in nickel-exchanged Ni-MCM-41 and Ni-AlMCM-41 in contrast to Ni/MCM-41, where Ni(II) was introduced into the synthesis mixture. The activity for ethylene dimerization decreases in the order Ni/MCM-41 > Ni-AlMCM-41 > Ni-MCM-41, which reflects the concentration of the catalytically active nickel(I) species. The product distribution is dominated by the acidity of the MCM-41 materials, which drives the isomerization of the primary reaction product 1-butene into cis- and trans-2-butene. Electron spin resonance studies show that an isolated Ni(I) species can be stabilized, which converts to a Ni(I)-C2D4 complex after ethylene adsorption. After subsequent heating to 343 K two new species are observed, which can be ascribed to Ni(I) product complexes.