For the preparation of an aqueous solution of unsaturated quaternary ammonium salts of the formula (I):H.sub.2 C.dbd.C--(R.sub.3)C(O)--A--R.sub.4 --N.sup..sym. (R.sub.1)(R.sub.2)(R).X.sup..crclbar.,e.g., methacryloyloxyethyltrimethylammonium chloridethe steps of:(a) reacting a (meth)acrylic monomer, e.g., dimethylaminoethyl methacrylate in a closed reactor, in the presence of a polymerization inhibitor, e.g., 3,5-ditert-butyl-4-hydroxytoluene, with 5 to 20% of the weight quantity of quaternizing agent, e.g., methylchloride, required for the reaction, wherein the latter agent is introduced into the reactor continuously,(b) adding water and the remainder of the quaternizing agent continuously until the required concentration of quaternary ammonium salt in water is obtained,(c) maintaining the temperature at a value of between 30.degree. and 60.degree. C. during the stages (a) and (b), and(d) maintaining a stream of gas containing oxygen in the reaction mixture during the stages (a) and (b) and especially as the end of the reaction approaches, such that the volume ratio of gas at the reactor outlet to the oxygen introduced at the inlet of this same reactor is less than 100/l.