Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.