A study has been made of the effect of the temperature and the substituent in the cyclopentadienyl ligand of the compounds (C5H4R)2Ni, where R = H, C2H5, n-C3H7, iso-C3H7 and tert-C4H9, and (C5H4R)N-C3H5, where R = H or iso-C3H7, on the kinetics and selectivity of ethylene dimerization reactions. It has been established that greatest activity is exhibited by (C5H4-iso-C3H7)2Ni. The influence of the Al/Ni molar ratio on the kinetics and selectivity of dimerization in the presence of the catalytic system (C5H4-iso-C3H7)2Ni-C2H5AlCl2 has been studied. The nature of the active centres formed when mono- and bicomponent catalysts containing nickelocene are used is discussed.