Reactions of vanadyl N and C-capped tris(phenolate) complexes 1 and 2 (Scheme 1) with AlEt3 and AlEt2Cl, in the absence and in the presence of ethyltrichloroacetate (ETA), were monitored by EPR. It was shown, that vanadium(IV) complexes with structures (LVIV)-V-N(Et)(AlEt3) and (LVIV)-V-N(Et)(AlEt2Cl) are formed at the initial stage of interaction of 1 with AlEt3 and AlEt2Cl, respectively (L-N are unidentified products of interaction of the initial N-capped ligand and V=O moiety with aluminum alkyls). In the absence of re-activator (ETA), (LVIV)-V-N(Et)(AlEt3) and (LVIV)-V-N(Et)(AlEt2Cl) rapidly convert at room temperature to V(III) species, which relatively slowly transform to complexes (LVEt2)-V-N-Et-II and (LVII)-V-N(Et)Cl. Concentration of (LVIV)-V-N(Et)(AlEt2Cl) dramatically increases in the presence of ETA, that correlates with the increase of the polymerization activity of the catalyst system 1/AlEt2Cl. The complex (LVIV)-V-N(Et)(AlEt2Cl) can be precursor of the active species of polymerization. Similarly, ETA increases concentration of vanadium (IV) complex (LVIV)-V-C(Et)(AlEt2Cl) in the catalyst system 2/AlEt2Cl (L-C are unidentified products of interaction of the initial C-capped ligand and V=O moiety with aluminum alkyls). The complex (LVIV)-V-C(Et) (AlEt2Cl) can be precursor of the active species of polymerization (C) 2008 Elsevier B.V. All rights reserved.