Polar photoisomers of bilirubin were formed by irradiation of bilirubin in chloroform solution in the absence of O2. Two pairs of compounds were isolated with molecular weights identical with bilirubin. One pair reverted to bilirubin in polar media and gave chemical reactions similar to bilirubin; the other pair were not reconverted into bilirubin by chemical means and gave reactions distinct from those of bilirubin. However, both groups were reconverted into bilirubin by irradiation in chloroform solution in the absence of O2. The probable role of these photoisomers in the catabolism of bilirubin during phototherapy of neonatal jaundice is discussed.