The structure of 12-tungstophosphoric heteropoly acid (H3PW12O40) supported on silica has been studied by Xe-129 NMR of adsorbed xenon. The Xe-129 NMR spectra are found to depend on the surface HPA concentration. The Xe-129 NMR data provide evidence for the presence of an organized microporous structure within HPA overlayers. These results are in agreement with nitrogen adsorption and capillary condensation (77 K) measurements. The high sensitivity of the Xe-129 NMR method and its applicability for testing of silica-based HPA catalysts are demonstrated.