This paper describes an illuminated reactor that allows the spontaneous development of biofilms aimed at the treatment of sulfide-containing streams. The reactor operates as a sulfidostat and is composed of an illuminated packed-column, in which microorganisms are exposed to constant low substrate concentrations, thereby avoiding inhibition due to high sulfide concentrations. The control system allows highly polluted streams to be oxidized by the microbial biofilm while ensuring the quality of the effluent produced. Both monospecies and multispecies biofilms have been developed. Biofilms undergo changes in light irradiance and sulfide load while providing a consistent reduction of the sulfide levels, down to micromolar concentrations. Both types of biofilm developed differ from stirred reactors in that their specific activities are lower, constituting systems with a slow dynamic behavior and, therefore, they are less sensitive to sudden disturbances. © Springer-Verlag 2004.