An environmentally friendly approach is presented to synthesize sulfonated reduced graphene oxide （S-rGO） by using L-ascorbic acid （L-AA） and aryl diazonium salt of sulfanilic acid. The preparation conditions have been optimized in order to obtain isolated and conductive S-rGO, and the products have been characterized by Ultraviolet-Visible spectroscopy, Fourier transformed infrared spectroscopy, Raman, X-ray photoelectron spectroscopy and X-ray powder diffraction and electrochemical methods. The results show that the S-rGO sheets possess excellent water-solubility and high electrical conductivity, which implies that the oxygen-containing functional groups have been removed and conjugated sp2 network has been restored. What＇s more, the electrochemical measurements reveal that the capacitive performance of the S-rGO has been improved compared with the graphene oxide （GO） and the reduced graphene oxide （rGO, reduced by L-AA）. The optimum S-rGO exhibits a specific capacitance of 205 F·g^-1 and good cycling stability （3.9% decreasing after 10000 cycles）, which are better than those for graphene oxide （109 F·g^-1 and decreasing 6.6% after 10000 cycles） and rGO （139 F·g^-1 and decreasing ll.3% after 10000 cycling）. This approach proves a new route to improve the capacitive properties of rGO.
