This paper aims to present the aspects of oxidation of pyrite and arsenopyrite in the sulphidic spoils in Lavrion and to assess the environmental impact of this process. Sulphidic spoils, under the combined action of atmospheric oxygen, water and bacteria are oxidised and toxic elements are released and migrate to surrounding areas contributing to widespread soil contamination and severe deterioration in the quality of surface and underground waters. The critical factors which control the generation of acidic waters are availability of oxygen and water, temperature, bacterial activity and rate of oxygen diffusion through the pores in the upper zones of spoils. Special emphasis will be given in the oxidation of sulphidic spoils at Lavrion, Greece, where the generation of acidic waters causes widespread contamination. This study is focused in the first part on the preferential oxidation of arsenopyrite. Flotation products containing pyrite and arsenopyrite were subjected to bacterial leaching tests at 3% w/v pulp density and the degrees of oxidation were calculated and compared to with previous experimental studies. These results provided the basis to assess the behaviour of the above minerals in waste stockpiles. Moreover, and in order to assess the oxidation and precipitation mechanisms, several samples were collected up to a depth of 20 cm from the surface from the same spoils profile, where successive layers of oxidation products were present and characterised chemically and mineralogically. The above information is critically evaluated for the determination of the environmental impact of the oxidation of sulphide phases in spoils.