An object is to prevent lowering in the yield of RCOF due to contamination by impurities and thereby to produce a high-purity product of RCOF in a stable manner. According to the present invention, provided are: a method of purifying a carboxylic acid fluoride, comprising a step of removing a hydrogen halide by bringing a carboxylic acid fluoride containing the hydrogen halide into contact with a metal fluoride; a method of producing a high-purity carboxylic acid fluoride, comprising a step of bringing a carboxylic acid fluoride containing a hydrogen halide into contact with a metal fluoride as well as a high-purity carboxylic acid fluoride obtained therefrom; and a method of using a metal fluoride as an adsorbent for a hydrogen halide in a method of removing a hydrogen halide from a carboxylic acid fluoride containing the hydrogen halide.

A method of purification of crude cis-2-alkenoic acid by thermal separation, comprising wiped-film evaporation or vacuum distillation of the crude cis-2-alkenoic acid, under a temperature-reduced pressure profile whereby isomerization of cis-2-alkenoic acid into trans-2-alkenoic acid is minimized.

A method of purification of crude cis-2-alkenoic acid by thermal separation, comprising wiped-film evaporation or vacuum distillation of the crude cis-2-alkenoic acid, under a temperature-reduced pressure profile whereby isomerization of cis-2-alkenoic acid into trans-2-alkenoic acid is minimized.