The amino acid Cysteine, which plays a significant role as a charge transfer bridge, e.g. in redox proteins like Ferredoxin or Mo-Nitrogenase and in artificial systems like Self Assembled Monolayers (SAM's) on gold, was adsorbed on both, natural and synthetic molybdenum disulfide and the changes were studied with combined photoelectrochemical and microwave conductivity techniques. In contrast to pyrite (FeS2), where modification with cysteine enhances the electrochemical corrosion, with molybdenum disulfide this is not the case. It was found that cysteine chemically interacts with both dangling bonds of molybdenum on edge sites as well as with d(z)(2) orbitals of molybdenum that protrude through the van der Waals surface. The Interaction with the edge sites leads to a decrease in dark current and hydrogen evolution activity. In inert electrolyte the interaction with the van der Waals surface leads to a decrease in decomposition-photocurrents due to the action of cysteine as a recombination centre for charge carriers. If, however, a reducing agent such as hydroquinone/quinone or hexacyanoferrate is added, photocurrents increase because the adsorbed cysteine now acts as charge transfer bridge and no longer as a recombination centre. This is supported by a significantly increased microwave conductivity indicating increased charge carrier lifetime.