Studies of the physical and chemical forms of I131 fallout following the USSR nuclear tests of late 1961 and the United States and USSR studies of 1962 have shown that the fraction existing in the gaseous state varied from about 10 to 90 per cent. Very little of the gaseous I131 appeared to be in the elemental or HI form. The I131 which could easily be leached from the particulate portion of the fallout was found to be mainly in the reduced form as iodides or elemental iodine; about one-third was in the iodate form and very little or none was in the periodate form.