The compatibility of poly(vinylchloride PVC) with post-chlorinated PVC (CPVC) is investigated with the purpose of relating the mechanical properties of such blends with their compatibility. The effects on compatibility of varying mixing methods, blend composition, molecular weights of components, and chlorine content of CPVC are studied. Experimental methods include comparison of the glass transition temperatures (Tgs) of blends and homopolymers using dynamic mechanical analysis, Differential Scanning Calorimetry (DSC), and comparison of the mechanical properties of blends and homopolymers. Experimental results indicate marginal compatibility between PVC and CPVC. Blends in which CPVC is the main component are indicated to be less compatible than other compositions.