We report on the internal modification of mesoporous silica (MPSi) by ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)), as an additive for the preparation of poly(ethylene oxide) (PEO)-based composite polymer electrolytes. The addition of EMImBF(4)-modified MPSi was effective for improving the ionic conduction in solid state. The room temperature conductivity was more than 3-fold enhanced by only 5 wt % addition of the modified MPSi as compared to the composite electrolyte with neat MPSi. (C) 2004 The Electrochemical Society.