Amorphous poly(ethylene oxide) (PEO)–SiO2 composites are prepared by in situ reactions that involve the simultaneous formation of the polymer network and inorganic nanoparticles. The polymer matrix is formed by ultraviolet irradiation of a PEO macromer, and silica is produced in situ by the sol–gel method. The PEO–SiO2 composite mixed with LiBF4 is used as a lithium-ion conducting solid electrolyte and electrochemical transport properties such as ionic conductivity and Li+ transference number are measured. A significant increase in the Li+ transference number, up to 0.56, is found together with a slight decrease in the ionic conductivity. The results are interpreted in terms of interactions between the surface OH groups of the inorganic particles, the cations, the anions, and the ether oxygen atoms on the PEO backbone.