The chain ends of polypropylene (PP) polymerized at 100 degrees C without molecular hydrogen addition over an MgCl2 . TiCl4 . dioctylphthalate/Et(3)Al/diphenyldimethoxysilane catalyst system consisted of ethyl and isobutyl groups. This indicates that the chain-transfer reaction in the production of this PP occurred mainly by Et(3)Al at 1,2-inserted sites. On the other hand, the terminal groups of PP obtained at 1000C with molecular hydrogen addition over this catalyst system were ethyl, isobutyl, n-propyl and n-butyl groups. The newly detected terminal groups were formed by chain-transfer reactions by hydrogen at 2,1-inserted sites. The observed activity enhancement in this catalyst system would be due to the conversion of dormant 2,1-inserted sites into the active sites by hydrogen addition.