Measuremnts of the quantum yield of methyl iodide (Ch3I) in seawater and other solvents were made in order to determine the potential of direct photolysis to compete to global surface waters with the decomposition of CH3I via SN2 (substitution, nucleophilic, bimolecular) reaction with Cl- ion. As a decomposition pathway it is potentially significant because photolysis does not yield methyl halides as products and may thus modulate the degree of supersaturation of surface water with respect to methyl chloride and methyl bromide. However the reduction in quantum yield (factor of 10) and the large hypsochromic shift (116 cm-1) for methyl iodide in going from nonpolar solvents to water reduces the importance of the direct photolysis in surface waters over most of the globe. Because of the temperature dependence of the SN2 reaction of Cl-, the halflike of CH3I increases with depth and latitude from 5-6 days in tropical surface water to 150 days at high latitude. This increase means that correlations in concentrations of CH3I and its decomposition products, CH3Cl and CH3Br, may be destroyed by either faster transport or primary production processes for CH3I.