IN dilute aqueous solution the oxidized prosthetic group of haemoglobin and catalase, ferriprotoporphyrin (the ferric iron derivative of 1,3,5,8-tetramethyl-2,4-divinylporphine-6,7-dipropionic acid) may be affected by ionizing radiation at several places in its complex structure. Dale and Russell1 and Barron and Johnson2 observed spectroscopic changes which led them to conclude that the porphyrin ring was broken. In addition, Barron and Johnson found that in the absence of oxygen the spectrum was changed to that of a ferroprotoporphyrin. We have obtained spectroscopic evidence that a new compound is produced before the ring is broken, and that the vinyl side-chains play an important part in its formation. It is also found that hydrogen peroxide will produce this compound.