Transparent and highly conducting (10(-3) S cm(-1)) composite gel electrolytes (cges) based on poly(methylmethacrylate) (PMMA) with 1 M LiClO4 in propylene carbonate (PC) and hydrophilic fumed silica (SiO2) added in different proportions up to 5 wt.% as the constituents were prepared. The unique property of the aggregates of surface hydroxyl groups of hydrophilic fumed silica to link together through hydrogen bonding to form a network and the interactions between various constituents, probed by FTIR spectroscopic technique, have revealed to control both the transport and theological properties of the cges. The high mechanical and electrochemical stability of these cges makes them potential candidates as electrolytes in "All Solid State Electrochromic Windows" (ECWs). Detailed room temperature conductivity and viscosity measurements correlated to FTIR spectroscopic analysis of cges along with their optical and electrochemical investigations is reported in the present paper. (C) 2004 Elsevier Ltd. All rights reserved.