The propene/ethene-[1-C-13] copolymerization method has been applied to measure precisely the regioselectivity of two modern MgCl2-supported Ziegler-Natta catalyst systems for the industrial production of isotactic polypropylene, namely MgCl2/di(isobutyl)-o-phthalate/TiCl4-cyclohexylmethyldimethoxysilane/AIR (3) (1) and MgCl2/2,2-di(isobutyl)-1,3-dimethoxypropane/TiCl4-AIR(3) (2). System 2 turned out to be slightly less regioselective than system 1 (on average, 0.26% 2,1 misinsertions instead of 0.18%), but the distribution of the regiodefects throughout the polymer was found to be more uniform, most likely as a result of a lower differentiation of the active species. This provides a simple explanation for the different response of the two catalysts to molecular hydrogen as a chain transfer agent. In system 1, indeed, part of the active species are almost completely regioselective, which means that H-2 finds very few preferential "cutting points" in the form of growing chains with a sterically hindered 2,1 last-inserted unit; therefore, a higher H-2 partial pressure is needed-compared with system 2-in order to achieve the same decrease of average polymer molecular mass.