The phenoldisulfonic acid method used for the measurement of nitrate-nitrogen in natural water is precise and has a good reproducibility, but the reaction of phenoldisulfonic acid with nitrate-nitrogen has not been clarified as yet. Spectrophotometric characteristics of the product from this reaction and optimal conditions for this test method were examined. The infrared absorption spectrum of the reaction product exhibits absorptions for NO₂, OH, and SO₃H. The nuclear magnetic resonance spectrum of the product indicated the presence of two H in meta position. The ultraviolet absorption spectrum indicated that this product is o-nitrophenol. From these spectral evidences, it was concluded as follows : 1) The product of reaction between phenyl-2, 4-disulfonic acid and nitrate-nitrogen is 6-nitrophenol-2, 4-disulfonic acid. 2) 6-Nitrophenol-2, 4-disulfonic acid produces an isosbestic point at 365 mμ with the addition of potassium hydroxide solution and the coloration becomes constant in alkalinity of above pH 6.5. 3) The solution colored by the addition of potassium hydroxide to 6-nitrophenol-2, 4-disulfonic acid shows stable absorption even when allowed to stand for a long time in day light. 4) Ammonia-nitrogen and cyanate ion do not interfere in the reaction of phenoldisulfonic acid and nitrate-nitrogen. Interference of nitrite-nitrogen can be removed by the addition of sulfaminic acid and that of sulfur ion by the addition of lead acetate.