A sensitive and accurate method for the determination of phosphate, involving the application of the Evelyn photoelectric colorimeter to the ceruleomolybdate reaction, is described. This technique makes it possible to differentiate phosphate phosphorus from other forms of phosphorus.Conditions affecting the rate and extent of color development have been studied. The results show that the maximum color intensity is developed m about five minutes after addition of the reducing agent. The determination may be made in the presence of extraneous color, and soil organic matter does not interfere with the reaction. Under the conditions specified, using light filters, Beer's law applies to the reaction in soil extracts as well as in pure solutions in the range 0.02 to 0.40 parts per million of phosphorus.