By selecting the solvent that dissolves interface activating reagents and bilirubin relatively well and after dissolving each the author prepared an interface activ bilirubin aqueous solution. This solution was evaporated at 80°C under a low pressure and the solubility in water and the chemical properties of the vapor were studied; and the following ressults were obtained. 1. Each interface activation reagent possesses its own specific capacity to dissolve bilirubin in water, and there is almost a direct-proportion relation between the quantity of bilirubin dissolved in water and the quantity of activating reagent required for such a dissolution. 2. The absorption curve of the bilirubin aqueous solution shows its maximum at 450 mμ, and with the increase in the quantity of the loaded activating reagent the absorption maximum moves 5-10 mμ towards the long side of the short wave; but in the case using non-ionic reagent the absorption maximum remains almost unchanged. 3. The bilirubin aqueous solution is positive to the direct diazo reaction, and its sensitivity to the diazo reaction is proportional to the quantity of the activating reagent added, making the maximum sensitvity to the diazo reaction stable beyond a given quantity. Therefore, the dissolution of bilirubin in water by the aid of the interface activating reagent and the positivity to the diazo reaction are two different phenomena. Thus it is impossible to say that the dissolution of bilirubin in water make it positive to the direct diazo reaction. 4. The bilirubin aqueous solution by the aid of interface activating reagent required 40 to 60 minutes for the completion of diazo reaction and the production of azo dye is about directly proportional to the time required for its production. 5. The absorption maximum of azo dye produced the diazo reaction always at 530 mμ even when pH of the solvent is changed, and the optical density increases proportionately to the strength of acidity. At the same pH the optical density at 530 mμ of azo dye produced by ionic activating reagent is higher than that of the azo dye produced by non-ionic activating reagent; whereas the optical density of 470 mμ on the long side of short wave, on the contrary, of the azo dye produced by non-ionic activating reagent is higher.