Disclosed is a composition of matter which provides heat and color stability to polyvinyl chloride resins, especially of the flexible type, said composition consists essentially of (A) about 10 to about 40 parts by weight of a zinc carboxylate; (B) about 50 to about 80 parts by weight of an alkyl ester of thiodipropionic acid; and (C) abqut 5 to about 20 parts by weight of a phenolic antioxidant. A PVC resin containing the above composition of matter is also disclosed.