The mechanism of a new color reaction of aminopyrine has been studied. Aminopyrine gives orange red coloration by the reaction with phenol and potassium ferricyanide in ammonium acetate or ammonium chloride buffer. Colored substance of this reaction has been isolated and determined to be 4-(1', 4'-benzoquinone-4'-imino) antipyrine, which is the same as the reaction product obtained from 4-aminoantipyrine. But in the reaction of aminopyrine it has been found that presence of ammonium ion is essential to the coloration, contrary to the reaction of 4-aminoantipyrine, and in presence of excess aminopyrine intensity of color is proportional to the concentration of ammonium ion. Further investigation of the role of ammonium ion in the color reaction of aminopyrine supports the suggestion that in the first step ammonium ion and phenol are oxydatively condensed to produce 1, 4-benzoquinone imide type intermediate, which reacts with aminopyrine to produce 4-(1', 4'-benzoquinone-4'-imino) antipyrine as the consequence of transdimethylaminaton. On the basis of these experiments, a new colorimetric determination method of ammonia has been established.