An acidic cellulose–chitin hybrid gel electrolyte consisting of cellulose, chitin, 1-butyl-3-methylimidazolium, 1-allyl-3-methylimidazolium bromide, and an aqueous H2SO4 solution is investigated for electric double layer capacitors (EDLCs) with activated carbon fiber cloth electrodes. The acidic cellulose–chitin hybrid gel electrolyte shows a high ionic conductivity comparable to that for an aqueous 2moldm−3 H2SO4 solution at 0–80°C. This system's temperature dependence in EDLC performance is investigated by galvanostatic charge–discharge measurement. An EDLC cell with the acidic hybrid gel electrolyte has higher capacitance than that with the aqueous H2SO4 solution in the range of operation temperatures (−10 to 60°C). Moreover, the capacitance retention of the EDLC cell with the acidic hybrid gel electrolyte is better than that of a cell with the H2SO4 solution at 60°C over 10,000 cycles. This suggests that the proposed acidic gel electrolyte has excellent stability in the presence of a strong acid, even at a high temperature of 60°C.