Reactions of thallium with water vapor and air were investigated in the range 25~176 to throw light on the mechanism of atmospheric corrosion. It was deduced on the basis of changes in the rate laws, and confirmed by x-ray diffraction, that the principal reaction product changes from T120 to T1OH as the relative humidity of air increases. Parabolic and cubic laws, as well as a new growth law, were observed. Effects of carbon dioxide and of preformed sulfide films were also studied. The tentative x-ray structure of T1OH is presented.