|Beschreibung:||Methyl iodide is an atmospheric trace gas and a major source of of atmospheric iodine. The ocaens are the main source of methyl iodide to the atmosphere, but the production pathways of methyl iodide in the ocean are poorly understood. In this thesis the surface seawater concentration of methyl iodide was measured together with air concentrations and the sea-air flux calculated from these measurements. The resulting net flux to the atmosphere was 22 nmol per square meter per day, giving a global oceanic source of 2.9 Gmol methyl iodide per year. Incubation experiments were done in the tropical Atlantic to get more information about the production pathway of methyl iodide in the ocean. The observed production rates were not influenced by filtering or poisoning of the seawater prior to the incubation, but strongly dependent on light. This points to a photochemical production parthway of methyl iodide. Simple model calculations show that the observed photochemical production may be the dominant source of methyl iodide in the ocean.