The oxidation reactivity of arsenopyrite FeAsS in dilute acid determines the recovery of gold from arsenopyritic gold ores by slurry electrolysis. Stoichiometric, sulfur- and arsenic- deficient arsenopyrite have practically the same oxidation rate. Arsenopyrite is covered with a metal deficient sulfide layer which appears to determine the restpotential of the electrode. This layer can be separately oxidized and reduced during bulk-oxidation of arsenopyrite. Arsenopyrite oxidation is accelerated by dissolved Na2SO4 and by high electrode potentials and solution acidity. The rate is controlled by a surface reaction with an apparent activation energy of 33 kJ mol(-1).