An electric double-layer capacitor (EDLC) was tested using load tests involving charging/discharging at 60°C for 17h using an electrolyte consisting of a solution of 1moll−1 of tetraethylammonium tetrafluoroborate (TEA·BF4) in sulfolane (SL) in order to examine the deterioration of the properties of the EDLC, including its capacitance. The interactions between the molecules of the electrolyte and the surface of the activated-carbon electrode were investigated by solid-state 13C-MAS-NMR spectroscopy. The molecules (ions) adsorbed on the surfaces of the positive and negative electrodes and the free molecules (ions) in liquid phase were clearly detectable by solid-sate 13C-MAS-NMR. In the blank spectrum, corresponding to the electrode prior to the load test, four pairs of sharp and broad peaks could be observed. Two sharp peaks from the liquid-phase SL appeared at about 24ppm and 52ppm, respectively, and those for the adsorbed molecules on the electrode surface appeared at about 17ppm and 45ppm, respectively, both being shifted to a higher magnetic field by about 7ppm. The peak shapes of the latter two adsorbed molecules were much broader than those in liquid phase, suggesting stronger interaction with the electrode surface and restriction of molecular movement. Two pairs of such sharp and broad peaks could also be observed for the TEA molecule, which contained two types of carbon atoms. The chemical shifts of the peaks of the adsorbed molecules gradually shifted to a lower magnetic field after the load test at higher voltages ranging from 0.7V to 1.3V. Such changes in the chemical shift of adsorbed molecules were intimately related to the weaker adsorption forces acting on the adsorbed molecules, indicating that the electrode surface had been deteriorated by the load test at higher voltages. On the basis of these solid-state 13C-MAS-NMR spectra, the deterioration mechanism of the EDLC electrode at higher temperatures was discussed.