Methyl iodide is known to be formed biologically in seawater and has been postulated as the agent of iodine air-sea transport. Kinetic calculations and experiments demonstrate that methyl iodide reacts with chloride ion in seawater to yield methyl chloride approximately as fast as it exchanges into the atmosphere. In seawater, both methyl chloride and methyl iodide are slowly hydrolyzed to methanol and halide ions. The rate of trapping of I on sea-salt particles by reaction with atmospheric methyl iodide is shown to be too slow to account for the enrichment of the marine aerosol in I relative to seawater....