The stability of supercapacitors is the key factor for their use under high temperature, high voltage and long-term durability. To improve the supercapacitor stability, there is a need to understand the degradation mechanism. In this work, the degradation sites in a carbon electrode at negative potential range are investigated in two common organic electrolytes: 1 M Et4NBF4 dissolved in propylene carbonate and in acetonitrile. To elucidate the common factor over a wide range of carbon materials, we examined eight kinds of carbon materials including activated carbons, carbon blacks, zeolite-template carbon (high surface area and a large amount of carbon edge sites) and graphene mesosponge (high surface area and a little amount of carbon edge sites). Their surface structures are distinguished into two regions: carbon basal planes and edge sites by nitrogen physisorption and high-sensitivity temperature-programmed desorption up to 1800 degrees C. Unlike the degradation at positive potential range, initial degradation reactions at negative potential range occur mainly on the carbon basal planes rather than the edge sites. This finding is corroborated by the theoretical calculation.