Nanoscale characterization of the acid properties of H3PMo12-xWxO40 (x = 0, 3, 6, 9, 12) was carried out by scanning tunneling: microscopy (STM) and tunneling spectroscopy, Pyridine binding with the acid sites of heteropolyacids (HPAs) was reflected in both the STM images and FT IR spectra of these materials: All the HPAs investigated formed well-ordered monolayer arrays on highly oriented pyrolytic graphite (HOPG) before and after pyridine adsorption. Exposure to pyridine increased the lattice constants of the two-dimensional HPA arrays by ca; 6:Exposure to pyridine also shifted the negative differential resistance (NDR) peak voltages of HPAs to less negative values in the::tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated:with acid properties of HPAs for the first time, suggesting that tunneling spectra measured by STM could serve to fingerprint acid properties of HPAs.