o-Phenylazophenols are known to chow strong intramolecular hydrogen handing and tautomerism.This paper descsribes a detailed study of the correlation of the intraruolocular hydrogen bonding and tautomerism with the structure of these compounds using various methods such as NMR spectra, ionizasion constant, mass spectra, infrared.spectra as well as perturbation molecular orbital(PMO) and Hückel molecular orbital (HMO) methods.