During the preparation of poly(vinyl chloride) (PVC) under low monomer pressures, intramolecular hydrogen transfers ("backbites") lead to the formation of small amounts of 1,3-bis(2-chloroethyl) branch (DEB) structures, as well as allylic chloride segments resulting from 1,6 shifts. The presence of these structural defects was established by the C-13 NMR spectra (recorded at 125.77 MHz) of dechlorinated PVC specimens made by organotin hydride reduction. At 55-80 degrees C, the two backbites leading to DEB groups differ substantially in relative rate, in that the backbiting:addition rate ratio is larger for the second backbite by a factor of 15-16, irrespective of temperature. No convincing evidence was obtained for the presence of a chlorinated 2-ethyl-n-hexyl branch (EHB) segment resulting from double backbiting by an alternative route. Backbiting effects on dichlorobutyl branch concentrations and on chain transfer to monomer are discussed and quantified, and the synthesis of models for dechlorinated DEB and EHB structures is described.