Methods for the precise, routine measurement of Ir in seawater, riverwater, and estuarine water using isotope dilution negative thermal ionization mass spectrometry (ID-NTIMS) have been developed. After equilibration with a (191)Ir-enriched spike, Ir is separated from solution by coprecipitation with ferric hydroxide, followed by anion exchange chromatography using a reductive elution technique. UV irradiation is employed for the decomposition of trace organics, which interfere with negative ion production. IrO(2)(-) ions are produced in the mass spectrometer by heating the sample on a Ni-wire filament in the presence of Ba(OH)(2). Detection efficiencies ranged from 0.1% to 0.3%. We have used these procedures to determine the concentrations of Ir in 4 kg samples from the Pacific Ocean, the Atlantic Ocean, the Baltic Sea, and the rivers supplying the Baltic. Our chemical procedures introduce a total blank of ∼2 × 10(8) atoms per sample. The distribution of Ir in the oceans is fairly uniform, averaging ∼4 × 10(8) atoms kg(-)(1). The concentrations in the rivers supplying the Baltic Sea range from (17.4 ± 0.9) × 10(8) for a pristine river to (92.9 ± 2.2) × 10(8) atoms kg(-)(1) for a polluted river. The distribution, speciation, and transport of Ir in natural waters can now be subjected to intensive study.