The influence of different factors on the kinetics and selectivity of ethylene codimerization with butadiene and isoprene catalyzed by FeAA(3) + AIR(3) systems in aromatic solvents was studied. The main products of these transformations were octa- and decatrienes, the codimers of ethylene with butadiene. The reactions between FeAA(3) and AlEt3 were studied in model conditions. The data on the yield and composition of gaseous products, as well as the structure of intermediates, were obtained. The assumption that active centers in ethylene codimerization and copolymerization with butadiene and isoprene are alkyl and hydride forms of univalent iron and complexes of zero-valent iron was proved. The scheme of the process suggesting the linear codimerization of ethylene with 1,3-dienes and isomerization of 1,4-hexadiene as a degenerated polymerization with univalent iron hydride as an active center was discussed in detail. The alternative mechanism of ethylene codimerization with 1,3-dienes in the presence or zero-valent iron complexes via the formation of a five-membered iron cycle was also discussed.