PURPOSE: To produce the subject compd. useful as an acylating agent in high purity from a carboxylic acid and inorg. acid halide by using a Mg halide or Mg in a form capable of being halogenated or a metal similar to Mg as a catalyst.

CONSTITUTION: A carboxylic acid (e.g.; p-acetoxybenzoic acid) and an inorg. acid halide (e.g.; phosgene) represented by the formula (wherein A is C or S and X is Cl or Br) are reacted by using a magnesium halide, a halide of a metal similar to magnesium (e.g. lithium, scandium) or a component becoming the magnesium halide or the halide of the metal similar to magnesium during reaction (e.g.; oxides or sulfates of these metals) as a catalyst to obtain a carboxylic acid halide (e.g.; p-acetoxybenzoyl chloride). The catalyst can be used again. A reaction by-product is little and a reaction accelerator is also unnecessary.