Complexes of the form (Tp*)MoOCl(P-OC6H4X) and (Tp*)MoO(p-OC6H4X)(2) (Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)-borate and X = OEt, OMe, Et, Me, H, F, Ci, Br, I, and CN) were examined by electrochemical techniques and gas-phase photoelectron spectroscopy to probe the effect of the remote substituent (X) on electron-transfer reactions at the oxomolybdenum core. Cyclic voltammetry revealed that all of these neutral Mo(V) compounds undergo a quastireversible one-electron oxidation (Mo-VI/Mo-V) and a quasireversible one-electron reduction (Mo-V/Mo-IV) at potentials that linearly depend on the electronic influence (Hammett sigma(p) parameter) of X. The first ionization energies for (Tp*)MoO(p-OC6H4X)(2) (X = OEt, OMe, H, F, and CN) were determined by photoelectron spectroscopy. A nearly linear correlation was found for the Mo-VI/Mo-V oxidation potentials in solution and the gas-phase ionization energies. Calculated heterogeneous electron-transfer rate constants show a slight systematic dependence on the substituent.