Nanoscale images and tunneling spectra of cation-exchanged or polyatom-substituted heteropolyacids (HPAs) were probed by scanning tunneling microscopy (STM) at room temperature. All the HPAs formed two-dimensional ordered arrays on graphite surfaces, and their molecular dimensions were in good agreement with values determined by X-ray crystallography. It was observed that the negative differential resistance (NDR) peak voltage measured by STM was closely related to the reduction potential of the corresponding bulk HPA. The higher the reduction potential of the HPA, the lower the applied voltage at which NDR was observed,