Analytical tools including solvent gradient elution fractionation (SGEF), GPC, C-13 NMR, and differential scanning calorimetry (DSC) are integrated for the characterization of compositional heterogeneity in the polyethylene (PE) prepared with the LFeCl2/AIEt(3) catalytic system. The results, indicate that at least two different kinds of catalytic species are preset-it in ethylene polymerization. One active species generating branched PE gives low molecular weight; another kind of active species gives high molecular weight PE with high linear structure. The amount of branch decreases with increasing the molecular weights, and the small proportion of the branched PE shows low molecular weight with vinyl-terminated end group, indicating that the branched PE is generated from the catalytic species giving low activity. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1151-1155, 2009