NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Acidified aqueous solutions containing [...], [...], and [...] have been irradiated by a high intensity flash lamp and the resulting change in the optical density of the solution has been observed spectrophotometrically as a function of time after the flash. Immediately following the flash, there is an increase in the optical density of the solution. This is attributed to the photochemical production of the diiodide ion, [...], by reaction (1) and/or reaction (2) and (3). [...] [...] [...]. The disappearance of the colored species, in about 1.5--8 milliseconds after the flash, has been found to be kinetically of the second order with respect to that species and has been explained in terms of the recombination of [...], principally by equation (4). [...] [...] [...]. On the basis of these assumptions, the value of [...] has been determined at wave lengths 365 mmu. and 435 mmu. for [...] ranging from [...] to [...] M. The experimental dependence of the rate of disappearance of [...] on [...] has been found to agree qualitatively with the theoretical equation: [...]. The calculated value of [...] has been found to vary with the wave length of the light used to observe the reaction as predicted by the equation: [...]. The presence of [...] in concentrations about [...] caused a slight increase in the value of [...], which was attributed to one or both of the reactions: [...] [...]. [...] in concentrations less than [...] appeared to decrease the rate of disappearance of [...]; for this anomalous effect no explanation is given. Experiments performed in which a large percentage of the light absorbed by [...] was cut off by filters produced correspondingly smaller phenomena but gave no conclusive evidence as to the relative proportion of [...] and [...] dissociated by the irradiation. An extensive summary of data is presented.