In this study, monoazo dyes were synthesized through the diazotization of several substituted aminobenzenes and couplings with benzene-1,3-diols. The chemical structures of these compounds were characterized using spectroscopic techniques. The tautomeric properties of the 4-(phenyldiazenyl)benzene-1,3-diol derivative compounds were examined using UV-Vis. spectrophotometry in pure polar or apolar media (dimethyl sulfoxide, ethanol, chloroform, benzene and cyclohexane) under acidic and basic conditions at 25±1 °C, 35±1 °C and 45±1 °C. The percentage of the azo-hydrazone tautomers was calculated for all of the compounds. The UV-Vis. analysis reveals that the azo form is dominant in the presence of azo-hydrazone tautomerism for all azo dyes. Concurrently, the acidity constants for these derivatives were determined using a UV-Vis. spectroscopic technique at 25 °C (0.1 °C).