(1) The absorption curve of indicator yellow was determined between 700 and 320 mμ over a range of hydrogen ion concentration from pH 5.2 to 10.0. The absorption maximum is at 420, 380 and 370 mμ at pH 5.2, 6.1 and 7.1-10.0 respectively. When the differential spectrum obtained by substracting the densi-ty of visual white from that of indicator yellow (representing the absorption of indicator yellow itself) was taken, the absorption maxima are at 440, 400, 380, 370 and 370 mμ at respectively pH 5.2, 6.1, 7.1, 8.2 and 9.0-10.0, and their heights are respectively 0.563, 0.500, 0.501, 0.743 and 0.810 when the absorption maximum of the visual purple is corrected to 1.000. The alkaline form has a narrower absorption band and the acid form a broader band. The form and height of the differential spectra at pH 9.0 and those at pH 10.0 are quite similar.(2) The change from indicator yellow solution to visual white solution by exposure to strong sunlight is more rapid in the acid form than in the alkaline form.