The mechanisms of chain epimerization during propylene polymerization with methylaluminoxane-activated rac-(EBTHI)ZrCl2 and rac-(EBI)ZrCl2 catalysts (EBTHI = ethylenebis(eta(5)-tetrahydroindenyl); EBI = ethylenebis(eta(5)-indenyl)) have been studied using specifically isotopically labeled propylene: CH2=(CDCH3)-C-13. These isospecific catalysts provide predominantly the expected [mmmm] pentads with [-(CH2CDCH3-)-C-13] repeating units (C-13 NMR). Under relatively low propylene concentrations at 50 and 75 degreesC, where stereoerrors attributable to chain epimerization are prevalent, 13C NMR spectra reveal C-13-labeled methylene groups along the polymer main chain, together with [(CDCH3)-C-13] units in [mmmr], [mmrr], and [mrrm] pentads and [(CHCH3)-C-13] units in [mmmmmm] and [mmmmmr] heptads, as well as [mrrm] pentads. The isotopomeric reglomisplacements; and stereoerrors are consistent with a mechanism involving beta-D elimination, olefin rotation and enantiofacial interconversions, and insertion to a tertiary alkyl intermediate [Zr-C(CH2D)((CH3)-C-13)P] (P = polymer chain), followed by the reverse steps to yield two stereoisomers of [Zr-CHDCH((CH3)-C-13)P] and [Zr-(CH2CH)-C-13(CH2D)P], as well as unrearranged [Zr-CH2CD((CH3)-C-13)P]. The absence of observable [-(CH2CHCH2D)-C-13-] in the [mrrm] pentad region of the C-13 NMR spectra provides evidence that an allyl/clihydrogen complex does not mediate chain epimerization.