The i.r. spectra of bilirubin isomers that differ in number and position of vinyl groups were examined to verify the assignment of the 988 cm−1 peak of bilirubin (991 cm−1 peak in calcium bilirubinate) to its pendant vinyl groups. There were only small changes in this peak with changes in position of vinyl groups (exo-2- and −18-vinyl versus endo-3- and −17-vinyl), but progressive loss of peaks in this region was observed when vinyl groups were reduced to ethyl groups (dihydrobilirubin and mesobilirubin). Methylvinylmaleimide, a monopyrrole derived from the outer (A and D) rings of bilirubin, has a pendant vinyl group and exhibits a prominent peak at 986 cm−1, but haematinic acid methyl ester derived from the inner (B and C) rings has no vinyl group and shows no peak near 988 cm−1. These observations verify the assignment of the 988 cm−1 peak of bilirubin to its pendant vinyl groups. This supports our previous proposal that a decrease in the peak at 985-995 cm−1 in the i.r. spectra of pigment gallstones, as compared with unconjugated bilirubin or calcium bilirubinate, indicates a consumption of vinyl groups in the process of formation of the polymer in the pigment stones.