A method is described for the determination of thallium in natural waters of low and high mineral salt contents. The sample is made 0.15 M in hydrobromic acid, filtered and after addition of bromine water it is passed through a column of the strongly basic anion exchange resin Dowex 1 on which thallium is adsorbed as the anionic thallium(III)-bromide complex. The thallium is eluted with an aqueous solution of sulphur dioxide and, after evaporation of the eluate, this element is determined by a.a.s. with an air-acetylene flame. The method was used successfully for the determination of thallium in waters suitable for drinking purposes, hydrothermal waters, sea-water and waters from oil-wells. © 1979, Taylor & Francis Group, LLC. All rights reserved.