Glycinethymol Blue (GTB) was purified chromatographycally with cellulose and ionexchange resin. Semi-Glycinethymol Blue was failed in purification because of its very insolubility in water. The acid formation constants of GTB have been investigated potentiometrically and spectrophotometrically. The formation constants of GTB are compared to those of the other Thymol Blue derivertives and the effects are discussed.