Amorphous cobalt sulfide (designated as Cos(x)) nanoparticles were first synthesized in the unique H2O/CS2 interface under mild hydrothermal treatment for 48 h. Electrochemical tests revealed that the high energy density accompanied by good power properties (a specific capacitance of 650 F g(-1) obtained at 4 A g(-1)) was delivered by the as-synthesized CoSx only after continuous 240 cyclic voltammetry (CV) cycles in 2 M KOH aqueous solution. which was in sharp contrast with its poor supercapacitive performance at the initial CV cycles. The energy-storage mechanism of the nanosized CoSx in the KOH solution is proposed as follows: During continuous CV cycling in the KOH solution, the phase transformation of as-prepared CoSx nanoparticles is slowly electrochemically induced into whiskerlike Co(OH)(2) around their surfaces, and it is the electrochemical reconstruction of whiskerlike Co(OH)(2) on their surfaces, rather than CoSx nanoparticles themselves, that are responsible for their good energy-storage energy-storage properties in the alkaline KOH electrolyte. (C) 2009 The Electrochemical Society.