Recent advances on substituent effects in transition metal bisarene complexes studied with high-resolution threshold ionization spectroscopy are reviewed to demonstrate new aspects of the ligand influence on electronic structures of sandwich molecules. Unprecedented accuracy in the determination of ionization energies provided by the laser techniques makes it possible to reveal and describe quantitatively such fine phenomena as isotope effects, the mutual substituent influence or variations of substituent effects on replacing the central metal atom with its Group analogues. In combination with DFT calculations, laser ionization spectroscopy unveils mechanisms of the ligand influence on unique redox properties of sandwich complexes which are of key importance for their practical use.