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A study has been made of the thermal stability of model compounds containing secondary and tertiary chlorine and also of unsaturated models which correspond to abnormal structures in poly(vinyl chloride). By considering in more detail the entropy changes taking place in the four-center hydrogen chloride elimination processes, we have applied and modified a simple, general entropy bond additivity method by which fairly reliable a priori estimates of the A factors for such reactions can be made. The activated entropies for elimination of hydrogen chloride were calculated on the principle of the additivity of bond entropies in the ground and in the fully developed cyclic-four-center transition state, respectively. On the basis of a few simple rules for assigning the corresponding bond entropies and entropy correction terms it is shown that quantitative estimates of A factors can be made to within the average experimental uncertainty of ±0.3 unit log A/sec-1. The extreme range of A values extends over ±0.5 in log A units. In a few isolated cases the discrepancy between estimated and observed A factors extends 0.9 log A unit and the weight of evidence suggests that the experimental values may not be reliable. In general, the entropic bond effect is a stabilizing character since it markedly decreases the unimolecular dehydrochlorination frequency factor in comparison with other model compounds, and so partially compensates the destabilizing energy effect of the internal bond on the activation energy.