The electrochemical behaviour of sulfate in Na2O . 2.5SiO(2) at 1000 degrees C is studied by means of cyclic voltammetry, square wave voltammetry and impedance spectroscopy. It appears that the thermal decomposition of sodium sulfate in Na2O . 2.5SiO(2) at 1000 degrees C is a complete and fast process providing a melt containing S(IV). The voltammograms show two reduction peaks attributed to the reduction of sulfite ions to elementary sulfur. The first peak corresponds to a reduction involving strong adsorption of sulfur on the electrode. The formation of a layer of adsorbed sulfur atoms during the prewave occurring anodic to the normal diffusion peak has a strong influence on the double layer capacitance measured by impedance spectroscopy.