Given the current technical standard of old rubber recycling and disposal, it is necessary to explore new technologies for recycling old rubber. Technologies with real material recycling are of special interest. The target of such technologies is the rebounding of ground and powdered old rubber in virgin rubber mixtures without deteriorating the mechanical properties of the final product. In most cases the surface of the particle is modified to allow for easier bonding between the particle and the virgin rubber. A new method for the surface modification of old sulfur cured, powdered rubber is the microbiological splitting and oxidation of the polysulfidic bridges in the rubber network. In this process the powder particles are treated by microorganisms under moderate conditions (30°C, ambient pressure) in an aqueous suspension. The biological process is restricted to the particle surface only. The treatment results in new functional groups on the old rubber particle surface and dissolved sulfate. A microorganism called Thiobacillus thioparus showed best desulfurizing results. The investigated types of rubber did represent typical sulfur cured mixtures of old rubber. The process desulfurized all used types of old rubber powders. The main parameters of the process were determined in this work. To compare the mechanical properties of virgin vulcanized rubber, with different fractions and types of treated and untreated rubber powder added, typical tests for mechanical properties such as elongation and tensile strength were performed. The evaluation of the test results shows that there is a correlation between the desulfurization of old rubber particles and good mechanical properties of the virgin rubber using the treated rubber powder. Spectroscopic methods were used to investigate the chemical structure and the behavior of chemical structure on particle surface. A concept for a commercial batch process was developed applying the established process parameters to reach the maximum degree of desulfurization. The specific cost of treated rubber powder is 1,160 DM per ton for a plant with a capacity of 10,000 ton per year. Thus, using an estimated sales price of 1,300 DM per ton, a plant with 6,000 tones per year can reach the profit zone.