A quaternary ammonium hydroxycarboxylate is produced by quaternizing a tertiary amine with a hydroxycarboxylic ester represented by the following Formula 1: ##STR1##

wherein R¹ to R³ are as defined in the disclosure. By using only one tertiary amine having at least one substituent different from R¹ of the ester moiety of the hydroxycarboxylic ester and by controlling the reaction conditions, a mixture of quaternary ammonium hydroxycarboxylates having different quaternary ammonium ions is produced. By reacting an inorganic acid with the quaternary ammonium hydroxycarboxylate or its mixture, a quaternary ammonium salt of inorganic acid or its mixture is produced.