PURPOSE: To obtain the compound useful as a size agent of paper or the like by the dehydrohalogenation reaction of an aliphatic fatty acid halide and a tertiary amine using a specified solvent with a smaller problem on health and environment.

CONSTITUTION: An alkyl ketene dimer is obtained by the dehydrohalogenation reaction of a 8-32C aliphatic fatty acid halide with a tertiary amine in an inert solvent. The solvent containing at least 30% particularly about 99 wt.% of oxygenated hydrocarbon chosen from a group of esters, ketones and aromatic ethers or their mixture is used. As the aliphatic fatty acid, a palmitoyl chloride is favorably used. As the tertiary amine, a triethylamine is favorably used. As the solvent, particularly a solvent chosc from an ester of ethyl acetate, ethyl propionate or the like, a ketone such as a methylethylketone, cyclohexanone or the like and an ether such as an anisole is favoranley used.