The Phillips CrOx/SiO2 catalyst is an important industrial catalyst for ethylene polymerization. However, understanding of the state of active sites and chain propagation mechanisms concerning the Phillips catalyst is still waiting for conclusive evidence. In this work, the Phillips CrOx/SiO2 catalyst, having been calcined, was used for investigating the copolymerization of ethylene and cyclopentene in the presence of triethylaluminum as a cocatalyst for the first time. The microstructures of the polymers were investigated with C-13-NMR and gel permeation chromatography methods. Because of the absence of internal double bond (C=C) in the copolymer main chain, the ring-opening metathesis polymerization of cyclopentene was excluded during the copolymerization stage of ethylene and cyclopentene. Also, the 1,2-insertion and 1,3-insertion of cyclopentene into the polyethylene main chain were confirmed. This evidence strongly implies that Cr=C species may not be the active sites for chain propagation; instead, the Cr-C active site model under the Cossee-Arlman chain propagation during the normal polymerization period. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1869-1877, 2009