The number of active centers (C-p) and propagation rate constants (k(p)) for ethylene polymerization with single-site homogeneous catalysts composed of cobalt (II) bis(imino)pyridine complexes: 2-'BuLCoCl2 and 2,5-'Bu2LCoCl2 (L: 2,6-(C6H3N=CMe)(2)C5H3N) and methylaluminoxane (MAO) as an activator have been determined by polymerization quenching with radioactive carbon monoxide. The above cobalt complexes provided formation of catalysts, characterized with an increased activity due to the increase in the C-p value. The values of propagation rate constant, determined for the studied catalysts, were close (3.6-2.4 x 10(3) Lmol(-1) s(-1)). The values of the constant of the chain transfer reaction with ethylene decreases sharply in the order 2,6-Me2LCoCl2 > 2-'BuLCoCl2 > 2,5-'Bu2LCoCl2, being the reason for the increase in the molecular weight of PE, produced with corresponding complexes. For the 2,5-'Bu2LCoCl2 + MAO catalyst, the effect of polymerization temperature on the C-p and k(p) values have been studied and the activation energy of propagation reaction was calculated (4.5 kcal mol(-1)), that was noticeably lower as compared with the effective activation energy (12.5 kcal mol(-1)), owing to the increase in the C-p value with polymerization temperature growth. (c) 2012 Elsevier B.V. All rights reserved.