The nature of Ni(II) and Ni(I) species formed upon the activation of the N,N-α-diimine Ni(II) precatalyst LNiIIBr2 (1) with AlMe3, MMAO, and AliBu3 has been studied by NMR and EPR spectroscopy (L = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diazabuta-1,3-diene). It has been shown that the paramagnetic complex 3AlMe3 with the proposed structure [LNiI(μ-Me)2AlMe2] or [(L•-)NiII(μ-Me)2AlMe2] predominates in the reaction solution of 1/AlMe3 at Al/Ni > 10. Upon activation of 1 with MMAO (Al/Ni > 20-30), nickel predominantly exists in the form of the structurally similar complex 3MMAO ([LNiI(μ-Me)2AliBu2] or [(L•-)NiII(μ-Me)2AliBu2]). The reaction of 1 with AliBu3 leads to rapid ligand scrambling, affording the EPR-active complex 4 ([(L•-)AliBu2]). The latter process is accompanied by Ni reduction to the zerovalent state. Possible roles of the observed species in the catalytic systems are discussed.