A study of factors affecting the ceruleomolybdate reaction for phosphate is reported. The concentrations of stannous and stannic ions, the presence of extraneous salts, and temperature are important factors affecting the intensity and stability of the blue colour.The stannous chloride reagent should be preserved from oxidation by storing it under hydrogen.The interference by ferric iron is due largely to its reaction with the reducing agent, and may be avoided by diluting the sample to contain 0.1 p.p.m. of phosphorus and using two or three times the usual quantity of stannous chloride.