The catalytic system Ni(COD)(2)/BF3 OEt2 has been studied in conversions of 1,5-cyclooctadiene under argon and ethylene atmosphere. It has been demonstrated that the catalytic system formed under argon exhibits a high effectiveness in cycloisomerization of 1,5-COD surpassing in this characteristic all known nickel complex catalysts (selectivity to bicyclo-[3 3 0]-octene-2 is up to 99 5% at 100% conversion). In the case of ethylene atmosphere the system produces mainly dinners (yield of cyclodimers above 70%) It has been shown that the catalytic system Ni(COD)(2)/BF3 OEt2 has the feature of "a living catalyst" consisting in resuming the initial activity with a new portion of 1,5-COD added when the monomer was fully exhausted The main and side products of the 1,5-COD conversion have been identified with GC-MS and preparative liquid chromatography combined with NMR and IR spectroscopy Based on EPR and IR spectroscopic data a mechanism for the catalytic performance of the Ni(COD)(2)/BF3 OEt2 system in argon or ethylene atmospheres is Suggested. It has been shown that Ni(0) is oxidized by the lewis acid to Ni(I) which is stabilized by substrate molecules in a mononuclear form without involvement of conventional organoelement entities Three sorts of paramagnetic nickel species have been found ionic complexes containing pi-coordinated COD ligands; ionic complexes sigma-bonded to COD: complexes as intimate pairs with BF4-counter tons. A mechanism for the catalytic conversion of 1,5-cyclooctadiene is proposed (C) 2009 Elsevier B.V. All rights reserved.