The microscopic structure of organic -inorganic hybrid membranes containing heteropolyacid has been investigated by atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and Fourier transform infrared spectroscopy (FT-IR). The hybrids were prepared by hydrolysis and condensation reactions, the so-called sol-gel reaction of 1,8-bis(triethoxysilyl)octane (TES-Oct) in the presence of phosphotungstic acid (PWA) with hydrated water. The characteristic lengths evaluated by using AFM and SAXS revealed the evolution of the domain structures upon increasing PWA concentration. Thermogravimetric analysis (TGA) indicated that increasing PWA concentration led to remarkable increase in the characteristic temperature for a weight loss. By combination of analysis using TGA and FT-IR, it was found that highly condensed domains were disintegrated, leading to the formation of loose networks with defect structures in the vicinity of the characteristic temperature. Correspondingly, the organic bridge was further broken. Variation of the characteristic temperature with the PWA concentration was strongly correlated to the evolution of the interdomain distance and the surface characteristic length scales, which were respectively observed by SAXS and AFM. Such a well-developed ionic domain structure offered fairly good performance of the proton conductivity in range 10(-4) to 10(-2) (S/cm) in spite of the rigid matrix of the sol-gel network.