Nanoscale investigation of Keggin-type heteropolyacid (HPA) self-assembled monolayers (SAMs) was performed by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to relate surface properties of nanostructured HPA monolayers to bulk redox and acid properties of HPAs. Cation-exchanged, polyatomsubstituted, and heteroatom-substituted HPAs were examined to see the effect of different substitutions. HPA samples were deposited on HOPG surfaces in order to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered monolayer arrays, and exhibited negative difference resistance (NDR) behavior in their tunneling spectra. NDR peaks measured for fresh HPA samples appeared at less negative potentials for higher reduction potentials of the HPAs. These changes could also be correlated with the electronegativities of the substituted atoms. Introduction of pyridine into the HPA arrays increased the lattice constants of the two-dimensional HPA arrays by ca. 6 Å. Exposure to pyridine also shifted NDR peak voltages of HPA samples to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs. This work demonstrates that tunneling spectra measured by STM can fingerprint acid and redox properties of HPA monolayers on the nanometer scale.