Abstract The concentration of iodate in seawater samples was determined at various pH values using differential pulse polarography (DPP). The sensitivity for the iodate determination decreased rapidly below pH 7 and above pH 9. The decrease below pH 7 may be caused by conversion of iodate to iodide, followed by the successive formation of I2 and I3. The decrease above pH 9 is probably due to the combination of iodate and hydroxide. Theoretical calculations have demonstrated that the optimum pH for iodate determination is above pH 7.4 for Po2 = 0.101 Pa.