analysis of the diagram demonstrates that under bio oxidation condition elemental sulfur was the main solid-phase product of arsenopyrite oxidation; it was formed early in the course of oxidation when the solution exhibited a relatively low Eh (0.2-0.3 v) with iron present mostly in the bivalent form and arsenous acid. During the subsequent deeper oxidation the pH of the liquid phase decreased from 1.6 to 1.4 whereas the Eh increased from 0.6 to 0.75 v. the resulting products of arsenopyrite oxidation were dissi=olved compouns