To study the effect of the silica content on the properties of the salt-free and salt-added hybrids based on poly(ethylene oxide) (PEO) and silica, two series of hybrids, PEO-silica and PEO-silica-LiClO4 (O:Li, 9:1) hybrids were prepared via the in situ acid-catalyzed sol-gel reactions of the precursors [i.e., PEO functionalized with triethoxysilane and tetraethyl orthosilicate (TEOS)I. The morphology of the hybrids was examined by scanning electron microscopy (SEM) of the fracture surfaces of the hybrid. The results indicated that the discontinuity develops with increasing the weight percent of silica in both hybrids. The differential scanning calorimetric (DSC) analysis indicated that effects of silica content on the glass transition temperatures (T,) of the PEO phase were different in salt-free and salt-added hybrids. The T, of PEO phase increased with increasing weight percent of silica in salt-free hybrids, whereas the curve of T, of PEO phase and silica content had a maximum at 35 wt % of silica content in salt-added hybrids. For both salt-free and salt-added hybrids, peaks of the loss tangent, determined by dynamic mechanical analysis (DMA) were gradually broadened and lowered with increasing weight percent of silica. The storage modulus, E', in the region above T,increases with increasing silica content for both PEO-silica and PEO-silica-LiClO4 hybrids. In the conductivity and composition curves for PEO-silica-LiClO4 hybrids, the conductivity shows a maximum value of 3.7 x 10(-6) S/cm, corresponding to the sample with a 35 wt % of silica.