DFT calculations are reported for all possible insertions of ethene and propene in rac-Me-2-SiInd(2)Zr-R](+) and Me2C(Cp)(Flu)Zr-R](+) (R = Et and iPr). The results confirm the basic stereoregulation mechanism of Corradini. In addition, they provide an ordering of the possible sources of stereoerrors: chain misorientation is the main mechanism for unbranched (Et, nPr) chains, whereas for the beta-branched iBu chain errors due to chain misorientation and chain-olefin syn orientation are equally likely.