Images and tunneling spectra of H3PMo12O10 and Cs3PMo12O40, deposited on graphite were obtained in air using scanning tunneling : microscopy before and after pyridine adsorption, and the effects of pyridine adsorption on the HPA arrays and on their tunneling spectra were examined. Fresh H3PMo12O40 formed well-ordered two-dimensional arrays with a nearly square symmetry on the graphite surface, with a characteristic lattice spacing of about 10.8 Angstrom, as determined from STM images. Upon interaction with pyridine, the STM images showed not only a change in array symmetry from square to hexagonal but also an increase in the lattice constant to ca. 16.5 Angstrom, as well. This indicated that even in the two-dimensional array of Hs(3)Mo(12)O(40), the similar to 6 Angstrom pyridine molecules were mainly adsorbed on the acid sites in the interstices between the polyanions, causing an increase in the interstitial spacing. In contrast, STM images of both fresh CS(3)PMO(12)O(40) and pyridine-adsorbed Cs3PMo12O40 showed similar two-dimensional ordered array formation with hexagonal symmetry and a lattice constant of about 13.7 Angstrom (reflecting the larger size of Cs+ than H+). This similarity of the surface arrays of Cs3PMo12O10 with and without pyridine exposure is due to the absence of acid adsorption sites for pyridine in the two-dimensional structure of Cs3PMo12O40. Confirmation of pyridine adsorption by H3PMo12O40, and not by Cs3PMo12O40 was also obtained by infrared spectroscopy and by spatially resolved tunneling spectroscopy; the latter technique distinguished the anion, cation, and void positions in these arrays.