Abstract: The average aggregation number (A) of tetrabutylammonium halides in organic solvent has been determined ebulliometrically. A of tetrabutylammonium bromide in benzene is much larger than that in chloroform (Fig.1), it seems that the polarity of the solvent is the major factor.In chloroform, A decreases in the order: iddide>bromide>chloride (Fig.2).When the quaternary ammonium salts exist as mixtures under phase-transfer reaction conditions, and A of the mixed bromide and iodide is determined and is found larger than each alone when there were no interaction between different ion pairs. We consider the ion pairs existing as mixture aggregate (see Table).