The scope of the book is to present the results of systematic investigations on the tautomerism of the prepared Azoisoxazolone dyes derived from the reaction of diazotized amine (aniline, 4-aminoantipyrine (4-aminophenazone), 2-aminothiazole and 3-amino-1,2,4-triazole) with 3-methyl-5-isoxazolone. The study of the experimental spectral and structural properties of these azo dyes as well as theoretical computational study of the tautomers of investigated compounds is further a goal of this research. This book contains the literature survey of previous works carried out on, the spectral and acid-base properties of azo dyes which derived from 3-methyl-5-isoxazolone, general introduction of Theoretical and Computational chemistry as well as biological activities of these azo dyes. Further more, the study includes the tautomerism, experimental and theoretical investigations of spectral properties as well as the spectrophotometric determination of the acid dissociation and association constants of the azo dyes under investigation.