Two examples of chemically modified gas diffusion electrodes are presented to demonstrate the value of electrode functionalization methods in the study of fuel cells. In one example, Os(2,2'-bipyridine)(3)(2+) has been added to the ionomer binder in the active layer of the electrode to measure the fraction of this ionomer that participates in ion transport within the layer. A lower fraction of the Os complex was electrochemically active in the cathode of an operating fuel cell, compared to a similar electrode that was in contact with an aqueous sulfuric acid solution. The difference is attributed to poor hydration of the fuel cell electrode. In a second example, a Ru complex has been covalently attached to a carbon-black fuel cell catalyst support to allow measurement of the fraction of the carbon surface that is in both electronic and ionic contact with the current collector and membrane, respectively.