To obtain a polyketone polymer composition stabilized against long- term thermal degradation in the final use and having useful melt stability by incorporating a polyketone polymer with a monophenol compound in a specified ratio.

This composition comprises a polyketone polymer and a monophenol compound represented by the formula, and wherein the monophenol compound is present in an amount of 0.05-5 wt.% based on the polyketone polymer. In the formula R1 and R2 are each hydrogen or a 1-6C alkyl; Y is a 3-15C carboxylic acid, carboxylic ester, ether or carbonamide group. The monophenol compound is a an alkyl hydroxyphenylalkanecarboxylate with its alkyl moiety being a saturated alkyl and may be interrupted by an ether bond and is exemplified by methyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate. In general, a polyketone polymer is used in a high-temperature service. The composition however, can give a final product, such as a fiber, a sheet a film a container or a pipe, stabilized against long-term thermal degradation.