A series of silica-polyvinyl alcohol (PVA) hybrids were prepared by acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) in the mixture of ethanol and water containing dissolved polyvinyl alcohol via a sol-gel process. A small-angle X-ray scattering (SAXS) study on those hybrid gels has been performed utilizing synchrotron radiation. Intensive positive deviation from Porod's law indicated that there existed density fluctuation within the matrix of silica-PVA hybrid gel. Using the differential d[q(3)I(q)]/ d(q(2)) as a variant slo, plotting ln(slo) versus ln(q) can produce the dimension n and the amplitude F-1 of density fluctuation. Density fluctuation resulted from both the micropores in gel and Si-O-PVA-O-Si bridge between SiO2 particle and PVA chain. When PVA increased in gel, the decreased micropores reduced the fluctuation but PVA enhanced the fluctuation. With the conjunction of correlation function C(r), it was clear that the bridge of Si-O-PVA-O-Si enlarged the compositional heterogeneity within gel matrix and made n increase with the increase of PVA. In addition, the fluctuation dimension n is a fraction rather than an integer that was formerly predicted by Ruland, which may relate to the fractal structure of hybrid gel. (C) 2003 Elsevier B.V. All rights reserved.