PURPOSE: To greatly improve the stability of a chlorinated resin by compounding it with a γ-diketone (salt).

CONSTITUTION: A chlorinated resin in an amount of 100 pts.wt. is compounded with 0.01-5 pts.wt. γ-diketone or its salt to give a stabilized, esp. thermally stabilized, chlorinated resin compsn. The γ-diketone is represented by the formula (wherein R1 and R2 are each H, an alkyl, an alkenyl, a cycloalkyl, an aryl, or an aralkyl provided they may be substd.; and R3 and R4 are each H or an alkyl). When a γ-diketone salt is used, it is pref. a salt of a lowly toxic and highly safe metal, esp. a divalent metal (e.g. Ca, Mg, or Zn). Since the compsn. is esp. resistant to heat and light, the degradation in the characteristics of the resin is inhibited. Moreover, the compsn. is lowly toxic and highly safe.