Ethylene polymerization reactions with many Ziegler-Natta catalysts exhibit several features which differentiate them from polymerization reactions of alpha-olefins: a relatively low ethylene reactivity, higher polymerization rates in the presence of alpha-olefins, a high reaction order with respect to ethylene concentration, and strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions (see ref 1) provided the basis for a new reaction scheme which explains all these features by postulating the equilibrium formation of a Ti-C2H5 species with the H atom in the methyl group beta-agostically coordinated to the Ti atom in an active center. This mechanism predicts that the beta-agostically stabilized Ti-C2H5 groups can decompose in the beta-hydride elimination reaction with expulsion of ethylene and the formation of a Ti-H bond even in the absence of hydrogen in the reaction medium. If D-2 is used as a chain transfer agent instead of H-2, the mechanism predicts the formation of deuterated ethylene molecules, which copolymerize with protioethylene. To prove this prediction, several ethylene homopolymerization reactions were carried out with a supported Ziegler-Natta titanium-based catalyst in the presence of large amounts of D-2. Analysis of gaseous reaction products and polymers confirmed the formation of several types of deuterated ethylene molecules and protio/deuterioethylene copolymers, respectively. In contrast, a metallocene catalyst, Cp2ZrCl2-MAO, does not exhibit these kinetic features. In the presence of deuterium, it produces only DCH2-CH2-(CH2-CH2)(x)-CH2-CH2D molecules.