We investigate by means of gas-phase ultraviolet photoelectron spectroscopy complemented by quantum-chemical calculations how the frontier levels of the 4,4'-bis(N-m-7tolyl-N-phenylamino)biphenyl (TPD) molecule are affected upon substitution of the terminal aryl rings with methoxy groups or fluorine atoms. These results provide strategies to modulate energy barriers at metal/organic or organic/organic interfaces involving TPD and its derivatives; it is shown that the change in the energy of the HOMO level of TPD upon derivatization is strongly affected by inductive effects taking place in the a skeleton.