A molecule-like substituent effect on redox formal potentials in the nanoparticle series Au-38(SPhX)(24) has been discovered. Electron-withdrawing "X" substituents energetically favor reduction and disfavor oxidation, and give formal potentials that correlate with Hammett substituent constants. The ligand monolayer of the nanoparticles is shown, thereby, to play a strong role in determining electronic energies of the nanoparticle core and is more than simply a protecting or capping layer. The substituent effect does not, however, detectably change the HOMO-LUMO gap energy, being identical for the HOMO and LUMO levels and presumably inductive in nature.