Polymerization of ethylene and copolymerizations of ethylene with 1-octene and oligoethylene having a vinyl end group were conducted with Cp 2 ZrCl 2 and [(C 5 Me 4 )Si-Me 2 N(t-Bu)]TiCl 2 a) as catalysts, and the structures of the resulting polymers were analyzed in detail. The Cp 2 ZrCl 2 catalyst combined with methylalumoxane (MAO) gives polyethylene with vinyl, vinylidene and trans-vinylene groups. The use of Al(iC 4 H 9 ) 3 /(C 6 H 5 ) 3 C.B(C 6 F 5 ) 4 as cocatalyst produces polyethylene predominantly containing trans-vinylene groups. Polyethylene with vinyl end groups was obtained selectively with [(C 5 Me 4 )SiMe 2 N(t-Bu)]TiCl 2 -MAO as catalyst. The types and contents of C=C double bonds in polyethylene markedly depend upon the metallocene compound as well as as the cocatalyst. In the copolymerizations the [(C 5 Me 4 )SiMe 2 N(t-Bu)]TiCl 2 -MAO catalyst system, on the other hand, gives poly[ethylene-co-(1-octene)] with a high content of 1-octene and a polyethylene containing an appreciable amount of long-side-chain oligoethylene.