Selecting solvent that dissolves interface activating reagents and bilirubin dimethylester relatively well and dissolving each of them the author prepared an interface active bilirubin-dimethylester aqueous solution. Then evaqorating this solution at 80°C under a lolw pressure, the auther studied the chemical proqerties of the vaqor dissolved in water and compare with the proqerties of bilirubin. The following are the results of this comparative study. 1. Each interface activating reagent possesses the specific capacity to dissolve bilirubin-dimethlyester in water, and there is a direct-proportion relationship between the quantity of bilirubin dimethylester dissolved in water and quantiy of activating reagent required for the dissolution. The bilirubindimethylester dissolving capacity of each reagent is 1.5 to 4.0 times the bilirubin dissolving capacity. 2. The absorption curve of the bilirubin-dimethylester aquoeus solution has its maximum at 420 mμ, and with the increase in the quantity of the activation reagent added, the maximum absorption curve moves 5-10 mμ towards the long side of short wave. However, in the case of non-ionic activating reagent hardly any such change occurs. 3. The bilirubin-dimethylester aqueous solution is always positive to the direct diazo reaction, and its sensitivity to diazo is not affected by the quantity of activating reagent but is fixed. 4. About 50-75 per cent diazo reaction of the bilirubidimethylester aqueous solution is completed within 5 minutes, but it requires about 40 minutes for its completion. 5. Spectroscopically there is no great difference between bilirubin-dimethylester azo and bilirubin azo, but the optical desnity at 490 mμ on the long side of short wave is higher in the former.