The structural changes in PVC during thermal dehydrochlorination in pyridine have been investigated. From gel permeation chromatography analyses, chain scission and cross-linking reactions were observed. Cross-linking increased with an increase in reaction time and temperature, whereas the chain scission increased with dilution of the reaction solution. Infrared spectros-copy indicated the presence of conjugated double bond sequences in polymeric chains. Polyene sequences were analyzed by ultraviolet spectroscopy. The length of the polyene sequences and the total number of conjugated double bonds increased with reaction time. An increase in dehydrochlorination temperature lowered the fraction of long polyene sequences, and thus the discoloration. Dilution of the reaction solution decreased the polyene sequence lengths as well as the total number of conjugated double bonds. Shorter polyenes were maximum and their proportion decreased with increase in their length.