In the photochemical degradation of bilirubin under anaerobic condition, the bilirubin riboflavin-system behaves as an equilibrium electrochemical system characterized by a redox-potential. Bilirubin decomposes irreversibly and riboflavin functions as a reversible electron acceptor. This equilibrium is not established under aerobic conditions, probably due to reoxidation of the riboflavin leuco-form by oxygen. Riboflavin accelerates the bilirubin photodegradation about 30 times at equimolar concentrations of the two components and in the presence of a buffer at pH 7.3. The present results of a study on a model system are applicable in the phototherapy of newborn infant hyperbilirubinemia.