The electrochemical oxidation of arsenopyrite (FeAsS) in 0.01 M chloride solution at pH 2 has been investigated and the effect of electrode potential, temperature and arsenopyrite mineral composition on the reaction stoichiometry studied. Iron, arsenic and sulfur products were formed in the ratio 1:1:1, for all conditions for arsenic deficient and stoichiometric arsenopyrite. Product speciation was dependent on temperature and potential, but not on arsenopyrite composition. At 25 degrees C, Fe(II), Fe(III), As(III), As(v), S, SO42- and S(x) (which could be a polythionate such as tetrathionate or pentathionate) were formed and similar to 9e(-) produced per mol of arsenopyrite oxidized. At 75 degrees C, practically no S(x) was formed and 7.5 e(-) produced per mol of arsenopyrite oxidized. A qualitative reaction scheme, based on the decomposition of thiosulfate to polythionate in the presence of As(III), is outlined.