The influence of ethanol and selected other alcohols on the formation of active nickel complexes for ethylene oligomerization was studied for the Ni(PPh3)(4)BF(3)center dot OEt2 system by EPR and H-1 NMR spectroscopy. Both the presence of Ni(I) complexes and Ni(II) hydrides could be established. Coordinatively unsaturated Ni(I) complexes were shown to be the active catalysts for ethylene oligomerization, whereas the Ni(II) hydrides were shown to be inactive. The alcohol act as a promoter for the Ni(PPh3)(4)BF(3)center dot OEt2 by facilitating the formation of the active Ni(I) complexes. (c) 2005 Elsevier B.V. All rights reserved.