The spectra and the changes in the electronic structure and in the properties of phenylazonaphthalenes and of azonaphthalenes by excitation have been calculated by thePariser-Parr-Poplemethod. Some relationships have been established, showing smaller changes by the transition from the ground state to the phosphorescence state than to the fluorescence one. Conclusions have been drawn for the influence of different substituents in the azobenzene (NH2, OH, C6H5) on the investigated variable.