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An addition of methanol into the aqueous solution of Chrome azurol S (CAS) accelerates the reaction of CAS with Cr. Mechanism of the accelerative effect of methanol was discussed spectrometrically. CAS has four chemical species, and the third chemical species (in which three protons released) is most liable to chelate Cr. In a high pH range, the third chemical species increases in the solution, however, at the same time, an increase in hydroxy ion interrupts the reaction between CAS and Cr. Results of spectrometrical measurements indicate that the added methanol accelerates the dissociation of the second chemical species of CAS in a low pH range and the reduction of the fourth chemical species in a high pH range. Consequently, the third chemical species is highly concentrated in the solution, that leads to accelerating the Cr-CAS reaction without any interruption caused on an increase of hydroxy ion.