A kinetic study of the photochemical and thermal conversion of photoisomers, especialy peaks 0 and 3 [(EE)-bilirubin IX alpha and (EZ)- and (ZE)-bilirubin IX alpha], under anaerobic conditions, was performed by using reversed-phase high-pressure liquid chromatography. Peaks 0 and 3 are spontaneously, photochemically and thermally converted. Short-term photoirradiation of bilirubin gives a mixture containing not only the geometric isomers (EE)-, (EZ)- and (ZE)-bilirubin IX alpha, but also peak 2, (EZ)-cyclobilirubin IX alpha. On prolonged irradiation, cyclization and 15Z leads to 15E isomerization lead to gradual accumulation of two pairs of photoproducts: (EZ)-cyclobilirubin IX alpha A and B and (EE)-cyclobilirubin IX alpha A and B.