A detailed study of the photochemistry of iodine and its oxides indicates that iodine species may play an important, but heretofore ignored, role in the tropospheric photochemical system. Methyl iodide, often observed in the marine troposphere with an average concentration of 5--10 ppt (v/v) is photolyzed and thereby produces I atoms. Chemical interactions with O3, HxOy, and NOx cause I to be converted to other inorganic compounds such as IO, HOI, IONO2, and I2. The production of these species and their subsequent recycling back to I can lead to the catalytic removal of tropospheric. O3, the enhancement of the NO2/NO ratio, the destruction of HxOy free radicals, and the conversion of HO2 to OH. Ultimately, tropospheric inorganic iodine (IX) is removed by heterogeneous processes. Calculations using a numerical model to simulate tropospheric photochemistry indicate that iodine may have a strong impact upon the atmospheric O3-NOx-HxOy system. The magnitude of these effects is dependent upon the value of several uncertain rate constants and the primary source distributions of CH3I and other organic and inorganic iodine compounds.