Differential scanning calorimetry was used to determine the miscibility behavior for the blends of polymethacrylates(PMA''s) having various alkyl groups with chlorine containing polymers. For the chlorine containing polymers, poly(vinyl chloride) (PVC), chlorinated po1yethylene(CPE) and chlorinated poly(vinyl chloride) (CPVC : 65wt.% and 68wt.% chlorine) were studied. The 1:1 blends of polymethacrylate containing ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, n-amyl, n-hexyl, and cyclohexyl group with PVC and CPVC were found miscible. The 1 : 1 blend of poly(methyl methacrylate) with PVC was found miscible. The 1 : 1 blend of poly(cyclohexyl methacrylate) with CPE was also miscible. It was found that the following blend were immiscible ; 1 : 1 blend of poly(methyl methacrylate) with CPVC, 1 : 1 blend of polymethacrylates containing glycidyl, 2-hydroxypropyl, t-butyl and phenyl group with PVC, CPE, and CPVC and 1 : 1 blends of polymethacrylates containing methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, and n-amyl group with CPE. In order to explain the miscibility behavior of the blends, the concept of the specific intermolecular interaction (SIMI) between two components was used. It was concluded that the miscibility window depends not only on the steric effect of the substituent group in PMA''s but also on the chlorine content and the conformation of chlorinated polymer.