The hydrogen activation effect in propylene polymerization reactions with Ti-based Ziegler-Natta catalysts is usually explained by hydrogenolysis of dormant active centers formed after secondary insertion of a propylene molecule into the growing polymer chain. This article proposes a different mechanism for the hydrogen activation effect due to hydrogenolysis of the Ti-iso-C3H7 group. This group can be formed in two reactions: (1) after secondary propylene insertion into the Ti-H bond (which is generated after p-hydrogen elimination in the growing polymer chain or after chain transfer with hydrogen), and (2) in the chain transfer with propylene if a propylene molecule is coordinated to the Ti atom in the secondary orientation. The Ti-CH(CH3)(2) species is relatively stable, possibly because of the beta-agostic interaction between the H atom of one of its CH. groups and the Ti atom. The validity of this mechanism was demonstrated in a gas chromatography study of oligomers formed in ethylene/alpha-olefin copolymerization reactions with delta-TiCl3/AlEt3 and TiCl4/dibutyl phthalate/MgCl2-AlEt3 catalysts. A quantitative analysis of gas chromatography data for ethylene/propylene co-oligomers showed that the probability of secondary propylene insertion into the Ti-H bond was only 3-4 times lower than the probability of primary insertion.