A common practice in mines and metallurgical plants around the world is to deposit tailings in decantation ponds. The chemical and microbiological transformations which occur in these ponds can severely affect the quality of the water. Furthermore, if these plants we SO2 as a flotation conditioner, the effluents become contaminated with sulphite ions. The present work was based on the study of a pond for pyritic tailings from the flotation of a complex polymetallic sulphide concentrate from the southwest of Spain. The chemical characterization of the pond waters was carried out by controlling pH, redox potential, conductivity and metals in solution. From a microbiological point of view, the study focused on screening those microorganisms directly related with oxidation and reduction processes: iron and sulphur oxidizing bacteria and sulphate reducing bacteria. The chemical oxidation of the sulphite ion and its influence on effluent quality was also studied. This ion produces reducing conditions in the system, which influence the chemical characteristics of the waters: pH, metals in solution, etc. In the absence of sulphite ions, different oxidative processes take over and microbial growth is greater.