Recent labeling experiments have detected stereoerrors in the isotactic polymerization of propene with zirconocenium catalysts, which are characterized by a deuterium transfer from the polymer chain to a methyl group. Using Cp2Zr-iso-butyl+ as a model system, two alternative mechanisms leading to the observed products are investigated using fully nonlocal levels of density functional theory (DFT). The direct hydrogen transfer starting from a γ-agostic intermediate requires an overall activation relative to the β-agostic resting state in excess of 200 kJ mol−1 and can be excluded. The sequential mechanism involves a (i) β-elimination to yield an initial iso-butene π-complex; (ii) a subsequent internal rotation of this π-complex; and (iii) reinsertion of the olefin into the ZrH bond via two consecutive formations of β-agostic interactions, with maximum relative energies along this path between 35 and 54 kJ mol−1 (depending on functionals and basis sets employed). The subsequent steps are the reverse of the first three steps and can lead to an inverted iso-butyl complex with migrated deuterium. Although certain barrier heights are somewhat sensitive to the density functional employed (BP86 vs. B3LYP), the sequential mechanism is much more favorable energetically. Loss of olefin from the intermediate π-complex is endothermic by 104–114 kJ mol−1 and should therefore not occur.