The process on the indirect diazo reaction after adding methanol to the bilirubinoids of bilirubin, mesobilirubin and bilirubindimethylester have been studied spectrophotometrically, the diazotised sulfanilic acid was used on the diazo reaction. The following results were obtained; Bilirubin-azopigment showed an absorption maximum at 560 mμ, and also at 420 mμ before the reaction was completed. The time to reach the absorption maximum at 560 mμ prolonged in parallel with a decrease of the amount of the diazo reagent, and also the absorption maximum at 420 mμ remained higher. The same results to the above were found in mesbilirubin-azopigment, except showing the absorption maximum at 550 mμ and at 390 mμ before the reaction was completed. Again the same results were found in bilirubindimethylester-azopigment, except showing the absorption maximum at 540 mμ and at 410 mμ before the reaction was completed. The absorption maximum of the bilirubin-azopigment at 560 mμ was flattened by leaving it in room, showing the more remarkable change along with a increase of amount of the diazo reagent. The same results were also found in the azopigments of the mesobilirubin and bilirubin dimethylester, but having slight differences in positions of the maximum absorption. After leaving the bilirubinoid in room, a maximum absorption was appeared at 400 to 420 mμ in the case having a plenty amount of diazo reagent, which was found to result from the excess amount of the reagent remained in the reacting solution. All of the acid azopigments, yielded after the addition of HCl to the each azopigments of bilirubin, mesobilirubin and bilirubindimethylester, showed their maximum absorption at 580 mμ, a very stable character and no significant change on the absorption curve by leaving them in room for a week.