Styrene and ethylene were copolymerized in the presence of a catalytic system based on CpTiCl3 and methylaluminoxane. 13C-NMR, FTIR, GPC and calorimetric techniques were employed to study the structure of the polymerization product. The 13C-NMR spectrum showed only three resonances which could be assigned to the methylene and methine protons of syndiotactic polystyrene and to the aliphatic carbon of polyethylene. Thus, the polymer was not a true copolymer but a mixture of polystyrene and polyethylene. Two melting endotherms and two molecular weight distributions were observed. In addition, a very low molecular weight (boiling butanone soluble) fraction was isolated. The existence of several polymerization products suggests the presence of more than one active species in the catalyst.