Abstract: A composition comprising a first polymeric component comprising chlorinated polyvinyl chloride (CPVC); a second polymeric component comprising at least one of (i) a mixture of styrene acrylonitrile (SAN) and an impact modifier, and (ii) a reaction product of the mixture; and a third polymeric component comprising alpha-methylstyrene acrylonitrile (AMSAN). The second and third polymeric components are present in a combined amount of at least about 7 parts by weight based on 100 parts by weight of the composition. The composition has a heat distortion temperature greater than about 180° F. according to ASTM D 648. A composite article comprises a capstock layer comprising a weatherable thermoplastic and a substrate layer comprising the composition.