The low excitation energy band appeared in the electronic spectra of some 4-hydroxyazobenzene derivatives is assigned to an electronic transition of the type n—π*. It is concluded that 4-hydroxyazobenzene derivatives exist mainly in the true azo form. On the other hand, the visible spectra of the azo compound; 4-hydroxy-2-carboxyazobenzene is interpreted on the basis that this compound exists in a tautomeric equilibrium of the type azo hydrazone, originating from the carboxyl group at the ortho position to the azo one.