A highly conductive composite electrolyte consisting of poly-2-hydroxyethyl methacrylate and a room temperature molten fluorohydrogenates, EtMeIm(HF)(n)F (EtMeIm: 1-ethyl-3-methylimidazolium, the averaged number n of the oligomeric anions in the vacuum stable salt is 2.3), has been obtained by a radical reaction method. A transparent gel composite electrolyte is formed when the molar fraction of EtMelm(HF)(n)F, N, is in the range of 0.20-0.60. The conductivity of the electrolyte increases as the N increases. The maximum conductivity of 2.3 x 10(-2) S cm(-1) is obtained for N=0.60 at 300 K. Like the original salt, this composite electrolyte exhibits an excellent stability in air and does not etch the Pyrex glass. The electrochemical window of the neat salt, similar to3.2 V, is extended to similar to3.5 V for the composite electrolyte.