The arc spectrum of iodine was first examined for fine structure by Wood and Kimura,* who failed to detect structure in any of the fines. The failure was probably due to the use of an unsatisfactory source and to the smallness of the resolving powers employed. The author then examined the iodine arc spectrum that was emitted by a high frequency electrodeless discharge in pure iodine vapour, using a variable gap Fabry-Perot interferometer as the resolving instrument. Structure was observed in eleven fines and an attempt at analysis was made. The nuclear spin of iodine was then unknown and, as only partial resolution was achieved, interpretation proved to be very difficult and ambiguous. Furthermore, at that time the existence of fine structure perturbations was not yet even suspected and, as will be shown later, one of the important s terms is perturbed. The nuclear spin of iodine was determined* from extensive measurements of the fine structures in the lines of the first spark spectrum. The lines produced in a water-cooled hollow cathode were investigated and, owing both to the sharpness of the individual components and to the relatively wider structures associated with the ionized atom, an unambiguous value for the spin was found (5/2). Knowing the spin with certainty, this can now be applied to the interpretation of the structures in the arc lines. However, realizing the superiority of the hollow cathode as a source, a complete re-examination of the spectrum was made. This revealed the fact that the previous measurements were all approximately correct, the differences being accounted for by the higher resolution now attained.