A process for preparing a high-purity quaternary ammonium salt comprising: (1) adding an oxide or hydroxide of a Group 1, 2, 12 or 13 metal 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 metal oxide or hydroxide to convert the acid salt to a tertiary amine and water and to convert the metal oxide or hydroxide to a metal salt at the same time, and (2) removing the tertiary amine, water and metal salt produced from the system.