Photoluminescence emission spectroscopy studies of heteropolyacid (HPA) catalysts have been carried out. All the HPA samples gave rise to a violet emission peak (emission peak I) at the same wavelength of 418 nm. However, a shoulder peak in the visible region (emission peak II) appeared at different wavelengths depending on the identity of the HPA sample. Photoluminescence emission peak (II) energies could be correlated with the negative differential resistance (NDR) peak voltages of nanostructured HPA monolayers measured using scanning tunneling microscopy (STM).