Adsorption of organic ligands [e.g., (1H-benzotriazol-1-yl)-methylisocyanide; btic, Fig. 1] and deposition of platinum on pyrite (FeS2) surfaces leads to the selective blocking of specific surface sites for electron transfer. In electrochemical experiments a drastic decrease of currents was observed with an Fe-2+/3+ electrolyte, but only slightly diminished currents for an l(-) electrolyte. From this it is inferred that the two different redox species interact with chemically different sites on the surface of pyrite.