Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes has been investigated using electrochemical techniques, in situ Fourier transform infrared spectroscopy (FTIR), ex situ X-ray photoelectron spectroscopy (XPS) and ion chromatography (IC). The results show that when pyrite is polarised in the potential region of - 0.35 to 0.25 V versus SCE in 1 M HCl at sweep rates greater than or equal to 30 mV s(-1), the redox process is a reversible electrochemical adsorption/desorption. whereas at sweep rates less than or equal to 10 mV s(-1) substantial oxidation of the pyrite surface occurs and becomes partially irreversible. The very limited oxidation rate of n-type pyrite over a wide range of potentials may be attributed to its semiconducting properties, or the main part of the applied potential falling across the space charge region rather than the Helmholtz layer. The electrochemical oxidation of pyrite involves a complex series and parallel reaction steps, ultimately producing Fe2+/Fe3+ and S/S2O32-/HSO4-, depending on pH and potential. A mechanism for this process is proposed and discussed using energy band and molecular orbital theories.