Non-aqueous gel electrolytes based on an alginate (Alg) and 1-ethyl-3-methylimidazolim tetrafluoroborate (EMImBF(4)) are prepared for electric double layer capacitors (EDLCs). The transparent colorless gel sheet has the high levels of mechanical strength and retentivity of ionic liquid needed to construct EDLC cells. According to charge/discharge measurements, a model EDLC cell with an Alg/EMImBF(4) gel shows high discharge capacitance in spite of using a gel-state electrolyte, implying that high affinity of alginate for an activated carbon electrode leads to a decrease in the electrode/electrolyte interfacial resistance. A test cell with Alg/EMImBF(4) maintains its coulombic efficiency over 99.8% during 5,000 cycles. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.020111esl] All rights reserved.