An improved ion chromatographic (IC)method was developed to determine trace levels of perchlorate in environmental water samples.Perchlorate was separated in the hydroxide selective column IonPac AS 16 using NaOH as an eluent with an organic modifier.To eliminate the coelution of perchlorate and 4-chlorobenzene sulfonate (4-CBS),an organic solvent as modifier was added to the eluent.Of four organic solvents studied,acetonitrile proved to be the most efficient based on the retention time of perchlorate and 4-CBS.To improve the method sensitivity,a concentrator column (AGI9) was used to concentrate perchlorate online.With the adoption of a preconcentration step,the sensitivity of our method was improved and the method detection limit (MDL) was reduced to 0.1 μg/L.The linear range was from 0.2 μg/L to 200/.μg/L with a linear correlation coefficient of 0.9989 and the relative standard deviation (RSD) of peak area for eleven successive injections of 0.5 μg/L perehlorate was 4.2%.The method had been applied to the determination of perchlorate in some real environmental water samples and recovery was between 93% and 113%.