Dimethoxybilirubin dimethyl ester and monomethoxybilirubin dimethyl ester were prepared by treating bilirubin with diazomethane, and the correctness of the assigned structures was proved by elemental analysis as well as by i.r. and n.m.r. spectroscopy. The phenylazo compounds derived from monomethoxybilirubin dimethyl ester were also prepared and characterized spectroscopically. Dimethoxybilirubin dimethyl ester occurs in solution as a single molecular species, unlike bilirubin dimethyl ester, which in non-polar solvents exists as an equilibrium mixture of conformational isomers. This difference in the behaviour of the two compounds is explained by the absence of intramolecular hydrogen bonds in dimethoxybilirubin dimethyl ester, a situation that allows free rotation about the central methylene bridge, whereas in bilirubin dimethyl ester an internally hydrogen-bonded conformation can be distinguished by n.m.r. spectroscopy from a non-bonded family of rotamers. This finding is regarded as additional evidence for a newly conceived conformational structure of bilirubin and bilirubin dimethyl ester that is maximally stabilized by intramolecular hydrogen bonds. This is discussed in detail in the Appendix (Kuenzle et al., 1973), which also includes a description of the molecular mechanism pertaining to the reaction of bilirubin with diazomethane.