Results pertaining to a chemically derivatized p-type semiconductor photocathode surface are reported. The reduced form of N,N'-dimethyl-4,4'-bipyridinium comes into rapid redox equilibrium with aqueous (pH < 6) solutions containing suspensions of Pt to evolve Hg; the H/sub 2/ evolution can also be catalyzed by hydrogenase. Included are results for derivatized Pt to establish the thermodynamics for the surface-confined reagents. Possible applications in bioelectrochemistry and in electrochronic displays with reversible electrodes functionalized with the bipyridinium reagent were noted. 2 figures. (DP)