Copolymers of vinyl chloride with propene, 1-butene, and 1-pentene containing 2–15 mol % of 1-olefinic structural units were prepared. The copolymers were dehydrochlorinated at 180°C in an inert atmosphere; the amount of hydrogen chloride split off was determined by continuous potentiometry. The results show that the heat stability of propene, 1-butene, and 1-pentene copolymers containing the same amount of 1-olefinic structural units does not differ significantly. Compared with the homopolymer of vinyl chloride, it is favorably affected by the presence of 1-olefinic structural units in poly(vinyl chloride) (PVC) chains. On the other hand, however, the heat stability of copolymers is impaired by the higher content of structural defects able to initiate the dehydrochlorination reaction. These structural defects, probably represented by chloroallyl groups, are formed in the copolymers during their synthesis. At the beginning of heating, structural defects produce intensive dehydrochlorination and, therefore, copolymers of vinyl chloride with 1-olefins if processed appear to be less thermally stable than does PVC.