The ultraviolet and visable absorption spectra of trans, cis-azoxybenzene, substituted azoxybenzenes and their conjugate acids have been studied. The 320-350$m{mu}$ main bands of free-base of azoxybenzenes are due to ${pi}{ o}{pi}^{ast}$ transition. These bands of their conjugate acids shown bathochromic shift into visible range. The following emperical relationship between absorption maxima of the main bands was found. ${lambda}_{max}={lambda}^{circ}_{max}+{Delta}{lambda}_x+{Delta}{lambda}_y$ This relationship in terms of wave number is also hold in good agreement. ${ u}={ u}_{circ}-({Delta}{ u}_x+{Delta}{ u}_y)$