Colorimetric determination of Phosphorus has been studied extensively and three reducing agents were tested: Some determinations of Phosphorus were carried out by reduction with stannous chloride. The deviation from Beer's law is considerable but we have used the Sols' tabulation for correcting the values. The amount of Phosphorus which can be accurately determined is 8-25 μg. Phosphorus has been also estimated by amidol and the color which it produce, is not as intensive as that obtained by stannous chloride. It shows a satisfactory degree of accuracy for Phosphorus contents between 2 and 75 μg. The values obtained by 1-amino-2naphtol-4-sulphonic acid, were slightly higher than those obtained by amidol. They gave a stable blue color proportional to the Phosphorus present. A slight modification of FISKE & SUBBAROW's method for the determination of Phosphorus during the intestinal absorption was worked out.