In polymers made from (E) or (Z)-[1-D]propene with methyl alumoxane (MAO)-activated ansa-zirconocene catalysts, signals of deuterium-labeled mrrm pentads document that stereoerrors arise mainly from an isomerization of the Zr-bound chain end. Reduced D-atom redistribution in poly([2-D]propene) indicates a kinetic isotope effect of k(H)/k(D) approximate to 3 for the beta-H/D transfer associated with the isomerization reaction. In all poly([1- and [2-D]propenes) studied, D-labeled mmmm pentads occur with a probability similar to that of D-labeled mrrm pentads; this observation requires further mechanistic clarification. Olefinic chain ends of polymers obtained with C2H4(thind)(3)ZrCl2-MAO from (E)- or (Z)-[1-D]propene deviate in their D-label distributions from the expected stereochemistry. Isomerization of the Zr-bound chain end, probably via a Zr-bound tertiary alkyl intermediate, thus contributes also to chain-growth termination. With the sterically hindered catalyst Me(2)Si(2-Me-4-tBu-C5H2)(2)ZrCl2-MAO, almost all chain terminations appear to occur vir chain-end isomerization, which does not lead to stereoerrors here. With the high-performance catalyst Me(2)Si(2-Me-benz[e]indmyl)(2)ZrCl2-MAO, finally, no D-label is found in the mrrm and mmmm pentad signals here, Zr-alkyl isomerization can apparently not compete with the high rate of olefin insertion.