Bulk redox properties of Keggin-type heteropolyacids (HPAs) were determined from surface properties of nanostructured HPA monolayers using scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). Cation-exchanged RnPMo12O40 (R = H+, Cs+, Ba2+,Zn2+, Co2+, Cu2+, Bi3+), polyatom-substituted HnPW11MO40 W = W6+, Mo6+, V5+) and heteroatom-substituted HnXW12O40 (X = p(5+), Si4+, B3+, Co2+) HPAs were examined to elucidate the effect of different substitutions. All HPA samples formed two-dimensional well-ordered monolayer arrays and exhibited negative difference resistance (NDR) behavior in their tunneling spectra. In all cases NDR peaks appeared at less negative potentials for higher reduction potentials of the HPAs. These changes could also be correlated with the electronegativity of the substituted atoms. Substitution of more electronegative atoms for counter-cations or central heteroatoms shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. However, substitution of more electronegative metals into the Keggin framework shifted the NDR peaks to more negative voltages, corresponding to decreased reduction potentials. (C) 2002 Elsevier Science B.V. All rights reserved.