Solutions of 2-diphenyl- or 2-dicyclohexylphosphinophenol, nickel bis(1,5-cyclooctadiene), and a tertiary phosphine R3P in toluene oligomerize ethylene in most cases with high selectivity to linear alpha-olefins. The molecular weights can be tuned by the catalyst composition. Molecular weights are generally lower with (Ph2PONi)-O-boolean AND than with cHex(2)P(boolean AND)ONi catalysts and in the presence of R3P additives, but basicity-dependent effects of various triarylphosphine additives on the molecular weights and relative reaction rates, displayed by pressure-time plots, are rather different or opposite for (Ph2PONi)-O-boolean AND and cHex(2)P(boolean AND)ONi catalysts. (C) 2004 Elsevier Inc. All rights reserved.