A sensitive method for rapid determination of inorganic phosphate in water samples is described which is based on a reaction between phosphomolybdate and resazurine; phosphomolybdrlte and p-rosolic acid. The molar absorptivities are 2867.5, 2656.9 at 495 and 470 nm with resazurine and p-rosolic acid reagents. Phosphate can be determirled in concentration range of 0.02 to 0.08 ppm, of 0.02 to 0.16 ppm with resazurine and p-rosolic acid reagents respectively.