AbstractFor the purpose of developing a sensitive and precise spectrophotometric method for uranium determination, the reaction of uranium (VI) with aluminon, ammonium salt of aurintricarboxylic acid, was rigorously examined. Uranium reacted with aluminon to give a pink color absorption complex with the maximum absorbance at 535 mμ and optimum pH of 4.0 to 4.8.The presence of two kinds of absorption complex, UO2++‐ATA and (UO2++)2‐ATA, was proved, the electromigration tests for both were shown to be negative. The molar extinction coefficient of UO2++‐ATA was found to be 7400 and that of (UO2++)2‐ATA, 12000, with the instability constants of 5.0×10−7and 2.8×10−12respectively. The practical sensitivity was 0.5 to 1 p.p.m. The color reaction was also found to conform with Beer's law, and the intensity did not change wit