The investigations described show that the formation of elemental sulfur from the biological oxidation of sulfide can be optimized by controling the redox state of the solution. The nonsoluble sulfur can be removed by gravity sedimentation and re-used as a raw material, i.e., in bioleaching processes. It was shown that, by supplying an almost stoichiometrical amount of oxygen to the recirculated gas phase, the formation of sulfate is minimized. The redox potential is mainly determined by the sulfide concentration because this compound has a high standard exchange current density with the platinum electrode surface. By maintaining a particular redox setpoint value, in fact, the reactor becomes a "sulfide-stat." It was shown that in a sulfide-oxidizing bioreactor the measured redox potential, using a polished redox electrode, is kinetically determined rather than thermodynamically. The optimal redox value for sulfur formation is between -147 and -137 mV (H-2 reference electrode, 30 degrees C, pH 8). The presented results are currently used for controling several full-scale installations, which desulfurize biogas and high-pressure natural gas.