Abstract: The potable water sample was digested with nitric acid, the sample solution was evaporated to a small volume and its acidity was adjusted to 1.7-2.0. Trace amount of Tl(Ⅰ) in the sample solution was oxidized to Tl(Ⅲ) by bromine water, separated on D401 ion-exchange resin column and determined by GFAAS. Under the optimum acidity around pH 1.8, Tl(Ⅲ) was separated from arsenic(Ⅴ), boron(Ⅲ), barium(Ⅱ), beryllium(Ⅱ), calcium(Ⅱ), gallium(Ⅲ), potassium(Ⅰ), lithium(Ⅰ), magnesium(Ⅱ), manganese(Ⅱ), sodium(Ⅰ), antimony(Ⅴ), silicon(Ⅳ), strontium(Ⅱ), bismuth(Ⅲ), copper(Ⅱ), titanium(Ⅳ), vanadium(Ⅴ) but was not separated completely from aluminium(Ⅲ), cadmium(Ⅱ), chromium(Ⅲ), iron(Ⅲ), nickel(Ⅱ), lead(Ⅱ) and zinc(Ⅱ). However, for most of the water samples, the amount of the above mentioned cations remained after the ion-exchange separation weren′t sufficient to interfere with the determination of thallium. The linearity range for Tl(Ⅲ) was between 25-500 pg, and the limit of quantification was 0.5 ng·L-1. The method was applied to the analysis of some substantial samples, giving results consistent with the known values. Recovery found by standard addition method were in the range of 94.0%-110% and RSD′s (n=8) found were in the range of 1.9%-15%.