Abstract: An improved photometric method for the determination of inorganic phosphate in fluids is disclosed.The method makes use of two reagents, an acid reagent and an ammonium molybdate reagent. Utilizing these reagents, the method comprises the following steps:(a) forming a first mixture consisting of a sample fluid and acid reagent;(b) measuring the light absorbance of said first mixture;(c) forming a second mixture consisting of said first mixture and ammonium molybdate reagent;(d) measuring the light absorbance of said second mixture;(e) calculating the difference in light absorbance (.DELTA.A sample) between the light absorbances measured for said first and second mixtures; and,(f) determining the concentration of inorganic phosphate in said sample fluid by comparing the difference in light absorbance (.DELTA.A sample) for said sample fluid to the difference in light absorbance (.DELTA.A standard) for a standard fluid having a known concentration of inorganic phosphate, .DELTA.