Using EPR spectroscopy, nickel(I) species formed in the catalyst system 1/MMAO for ethylene polymerization has been investigated (1 = LNiBr2, L = 1,4-bis-2,4,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-dien). It has been shown that diamagnetic cationic complex [LNiII-tBu]+[MeMMAO]– (2) persists in system 1/MMAO at low temperatures, but rapidly decays at 25 °C to give new EPR active species 3 (major product) and 4 (minor product). Species 3 is a nickel(I) complex with the proposed structure [LNiI(S)]+[MeMMAO]– (S = solvent), whereas 4 is L(•−)AlMe2 species. Upon the addition of ethylene, complex 3 partially reversibly converts into the adduct [LNiI(C2H4)]+[MeMMAO]– (5). After ethylene consumption, species 3 restores its original concentration.