A new method for the determination of low concentrations of chlorate in natural waters is described. Chlorate is analyzed by ion-chromatography followed by an osmate-catalyzed postcolumn reaction of chlorate with iodide and UV-detection of triiodide. The new osmate catalysis allows to carry out the oxidation of iodide by chlorate at pH 3 instead of 6 M HCl for the uncatalyzed reaction. A detection limit of 5 nM (0.4 mu g/l) chlorate is achieved. The method also allows the simultaneous determination of chlorite, bromate, and nitrite at the low mu g/l level. (C) 1999 Elsevier Science B.V. All rights reserved.