Poly(vinyl chloride) (PVC) was chlorinated with or withoutCHCls as swelling agent, to provide samples of CPVC for study. The following properties were determined: thermal stability (dehydrochlorination), Vicat softening point, dynamic modulus, impact strength at various temperatures, and creep at 80° and 100°C. All these properties depend on both chlorine content and type of chlorination process. At constant chlorine content the CPVC samples prepared in the presence of the swelling agent have higher thermal stability, higher Vicat softening point, lower impact strength, and lower creep deformation than the samples prepared without the swelling agent. The logarithm curves of dynamic modulus vs. temperature of the blends of PVC with CPVC of high chlorine content, exhibit a sharp flex at the glass transition temperature of PVC. Chlorination without a swelling agent results in a blend between the PVC and CPVC of various chlorine contents. Moreover PVC and CPVC with high chlorine content are incompatible. When mixed together, heterogeneous blends are obtained."pvuring the past few years many studies have been made to find an industrial way to obtain postchlorinated poly (vinyl chloride) (CPVC). The processes which, for simplicity and low cost, seem most promising are those in which PVC is chlorinated in the heterogeneous phase. These processes can be carried out either in the presence or ab sence of a liquid-dispersing phase and with the PVC either swollen by a solvent (swollen process or gel phase) (4,14) or in the unmodified powder state (unswollen process) (7, 21).