Arsenopyrite is commonly present in mining tailings and, together with pyrite, is responsible for acid rock drainage (ARD) phenomenon. The mineral undergoes oxidation in contact with oxygen and water producing a solution containing acid and heavy metals and, hence, causing important environmental impacts. Density functional/plane wave calculations were carried out to investigate the oxidation of arsenopyrite aiming to understanding its intricate mechanism at a molecular level. Molecular oxygen is dissociatively adsorbed in the Fe-O-As adsorption sites leading to the oxidation of arsenic and iron sites in good agreement with the available experimental data. It avoids the many steps observed for the pyrite oxidation mechanism. The presence of water is extremely important for the next steps of the oxidation mechanism similarly to the oxidation mechanism of pyrite. The present work reinforces the fact that the removal of the humidity can inhibit the oxidation of the arseno(pyrites) in an aerobic condition.