A process for preparing a highly pure quaternary ammonium salt comprising: (1) adding a quaternary ammonium hydroxide salt or quaternary ammonium carbonate to a quaternary ammonium salt containing a protonic acid salt of a tertiary amine as an impurity and thereby neutralizing the tertiary amine protonic acid salt with the quaternary ammonium hydroxide salt or carbonate to convert the acid salt to a tertiary amine and water and to convert the quaternary ammonium hydroxide salt or carbonate to a quaternary ammonium salt at the same time, and (2) removing the tertiary amine and water produced from the system.