Novel bipyridyl-osmium complex conjugates and their use in electrochemical assays are described. The redox reversible-osmium complexes can be prepared to exhibit unique reversible redox potentials and can thus be used in combination with other electroactive redox reversible species having redox potentials differing by at least 50 millivolts in electrochemical assays designed for use of multiple electroactive species in the same cell and in the same sample without interference between the two or more redox coupled conjugate systems.