Suspension poly(vinyl chloride) (PVC) samples with various monomer conversions were synthesized using batch and semi-batch polymerization processes at different temperatures. The concentration of tertiary chlorines in the PVC was determined using C-13 nuclear magnetic resonance, and the concentration of internal double bonds was analysed by ozonolysis. The dehydrochlorination rate of the PVC was measured at 190-degrees-C by the conductimetric method under nitrogen environment. It was found that the concentration of tertiary chlorines increases significantly with increase in monomer conversion after the critical conversion at which the reactor pressure starts to fall. At the same conversion level, it increases with polymerization temperature. The concentration of allylic chlorines is much lower than that of tertiary chlorines. A conversion dependence of allylic chlorines was not found for the present PVC samples. However, the concentration of allylic chlorines increases with polymerization temperature. Excellent correlation between the dehydrochlorination rate and the concentration of tertiary chlorines was found based on the present experimental data, but no significant relationship was found between dehydrochlorination rate and concentration of allylic chlorines. Therefore, tertiary chlorines in PVC chains are probably the defect structure that is most responsible for the reduced thermal stability of PVC. The mechanisms of formation of internal defect structures and the effects of polymerization conditions on the concentration of the defect structures are discussed in some detail based on diffusion-controlled free-radical polymerization theory, and the monomer concentration effects are further confirmed by the results using a semi-batch polymerization process. The concentration of tertiary chlorines can be minimized by semi-batch operation at or near the saturation pressure, providing PVC of higher thermal stability.