The propene/ethene-[1-13 C] copolymerization method has been applied to measure precisely the regioselectivity in propene polymerization of the "simple" MgCl2/TiCl4 catalyst (activated with Al( i -Bu)3), which can be viewed as the ancestor of all present-day MgCl2-supported industrial catalysts containing Lewis bases as selective modifiers. It has been found that the average content of 2,1regiomistakes in the polypropylene produced is fairly high (ca. 0.7 mol %), althoughsnot unexpectedly, due to the multisite catalyst naturesthese are distributed rather nonuniformly throughout the polymer. The difficulty in detecting the 2,1-units, despite their relatively high concentration, in the 13 C NMR spectra of homopolymer samples at natural 13 C abundance has been traced to their occurrence in a variety of stereochemical environments, which leads to splitting and broadening of the corresponding resonances. The fraction of "dormant" polypropylene chains with a 2,1-last-inserted unit was estimated to be in the 20-50% range. QM calculations of regioselectivity on classical Corradini-type models of epitactic octahedral catalytic Ti species turned out to be in basic agreement with the experiment.