Lake Constance is one of Europe s largest oligotrophic lakes and provides a water source for more than 4.5 million people in Germany and Switzerland. We present here a 12 month study on iodine concentrations, speciation and fluxes to and from the lake to gain a quantitative understanding of the limnic iodine cycle. Monthly water samples were obtained from all major tributaries (14) and the outflow to construct a mass-balance model. Sediment traps were also deployed in the lake for two years at two different stations. Total soluble iodine (TSI) in aqueous samples were analysed by ICP-MS and speciation (iodide, iodate and soluble organically bound iodine, SOT) by ion chromatography-ICP-MS. Iodine concentrations in the Alpine tributaries (1-2 mu g l(-1)) decreased over the summer months due to increasing proportions of snow and glacial melt water from the Alps, while iodine levels in the lowland rivers (similar to 2-10 mu g l(-1)) increased over the summer. Deposition of TS1 to the catchment (16,340 kg I yr(-1)) was similar to the TSI out-flux by rivers (16,000 kg I yr(-1)).