High grade quaternary ammonium salts represented by the following General Formula II: ##STR1## (in which R.sub.1 represents a hydrogen atom or a methyl group; R.sub.2 and R.sub.3 independently represents an alkyl group containing 1 to 4 carbon atoms; A represents an oxygen atom or an --NH-- group; B represents an alkylene group containing 1 to 4 carbon atoms; R.sub.4 represents an alkyl group or a benzyl group; and X.sup.- represents a halogen anion) can be produced by reacting a cationic vinyl monomer represented by the following General Formula I: ##STR2## (in which R.sub.1, R.sub.2, R.sub.3, A and B have the same meanings as defined above) with an alkyl halide or an aralkyl halide in a medium consisting of a particular mixture of water and an aprotic organic solvent under a condition where the concentration of oxygen dissolved in the reaction system is lower than 1.8 ppm but not less than 0.3 ppm. The reaction proceeds quite smoothly, without forming deposition of quaternary ammonium salts produced by the reaction. The overheating of the reaction system, which is a cause for the generation of gelation-causing substances and polymerization inhibitors, can be prevented effectively since the reaction mixture can be smoothly stirred and hence the generated heat can be efficiently removed therefrom. The process therefore enables to produce on a commercial scale aqueous solutions of unsaturated quaternary ammonium salts of very high quality which can be used as a raw material for producing cationic polymers of high molecular weight.