Detailed GC analysis of oligomers formed in ethylene homopolymerization reactions, ethylene/1-hexene copolymerization reactions, and homo-oligomerization reactions of 1-hexene and 1-octene in the presence of a chromium oxide and an organochromium catalyst is carried out. A combination of these data with the analysis of C-13 NMR and IR spectra of the respective high molecular weight polymerization products indicates that the standard olefin polymerization mechanism, according to which the starting chain end of each polymer molecule is saturated and the terminal chain end is a C=C bond (in the absence of hydrogen in the polymerization reactions), is also applicable to olefin polymerization reactions with both types of chromium-based catalysts. The mechanism of active center formation and polymerization is proposed for the reactions. Two additional features of the polymerization reactions, co-trimerization of olefins over chromium oxide catalysts and formation of methyl branches in polyethylene chains in the presence of organochromium catalysts, also find confirmation in the GC analysis. (C) 2008 Wiley Periodicals, Inc.