A method of ion-exchange chromatography with direct conductivity detection to determine three homologous quaternary ammonium cations(tetramethyl-ammonium cation,tetraethyl-ammonium cation and tetrapropyl-ammonium cation) was developed Three homologous quaternary ammonium cations were separated on a sulfonic acid base cation-exchange column using ethylenediamine-citric acid-acetonitrile as eluent.Effects of eluent types,eluent concentrations and column temperatures on retention and separation of three cations were discussed.The separations of the cations were improved by changing concentration of ethylenediamine and acetonitrile in eluents.With the increase of acetonitrile concentration,the retention time of tetrapropyl ammonium cation was obviously reduced,and the chromatogrophic peaks of the cations were improved.The retentions of homologous quaternary ammonium cations accord with carbon number rule.The suitable eluents for analysis of tetramethyl-ammonium cation and tetraethyl-ammonium,tetrapropyl ammonium cations were 0.02 mmol/L ethylenediamine-0.12 mmol/L citric acid(pH 4.0) and 0.2 mmol/L ethylenediamine-0.4 mmol/L citric acid-1% acetonitrile(pH 4.0),respectively.The flow rate and column temperature were 1.0 mL/min and 40 ℃,respectively.The detection limits(S/N=3) for three cations were 0.015,0.22,1.88 mg/L,respectively.Relative standard deviations(n=5) for peak areas were less than 2.3%.The method was successfully applied in the determination of quaternary ammonium ionic liquids synthesized in chemical laboratory and surfacetant tetramethyl-ammonium hydroxide,with recoveries of 99%-104%.The method is simple,accurate,reliable and practical.