Composite electrolyte comprising methylsisesquioxane (MSQ) filler and 1-butyl-3-methyl-imidazolium-tetrafluoroborate (BMImBF(4)) ionic liquid (IL) in poly(2-hydroxyethyl methacrylate) (PHEMA) matrix had been prepared. The polymer matrix was formed by free radical polymerization of HEMA macromer, and MSQ was produced in situ from methyl-trimethoxysilane by the sol-gel method. Infrared spectroscopy and dynamic mechanical analysis were employed to give insight into the interactions among the methylsisesquioxane filler, BMImBF(4) and the PHEMA polymer matrix. The PHEMA-IL-MSQ hybrids and the PHEMA-IL electrolyte without MSQ were investigated regarding their ionic conductivity and thermal and electrochemical properties. BMImBF(4) increased the thermal stability of the polymer and provided the ion conductivity MSQ filler as the additive increased the mechanical strength of the polymer and provided the ion conductive pathway. The electrolyte with MSQ at the 10 wt% showed the highest ionic conductivity of 5 X 10(-4) S cm(- 1) which was five times higher than that of the electrolyte Without MSQ and the electrochemical window was Lip to 3.6 V. (c) 2005 Elsevier Ltd. All rights reserved.