In a paper on pyromeconic acid read before this Society, and since published in the Philosophical Magazine for September 1852, I have detailed the preparation and properties of a compound obtained by the substitution of an equivalent of bromine for an equivalent of hydrogen in that acid. Having observed that this substitution was very easily effected, I was induced to attempt the formation of an iodopyromeconic acid, in the hope of adding one to the very few instances in which the direct substitution of iodine for hydrogen has been found possible. For this purpose, I digested pyromeconic acid with tincture of iodine, but no success attended the experiment, the acid remaining entirely unchanged. The failure of this attempt led me to speculate as to its cause, and to contrive a method of producing the required substitution which has proved entirely successful, and has the further advantage of being perfectly general, so that its application will enable chemists to obtain iodine substitution compounds in cases in which they have hitherto failed.