Abstract The chemisorption and electrocatalytic reactivity of 3,6-dihydroxypyridazine (DHPz) has been compared at smooth polycrystalline platinum and gold electrodes in aqueous acid and neutral solutions. Analytical measurements of packing densities and electrochemical reactivities were based upon thin-layer electrochemical techniques. At ambient temperatures, DHPz irreversibly adsorbs on Pt via only one nitrogen, regardless of pH. This means that the driving force for coordination of the nitrogen heteroatom to the Pt surface is greater than its protonation, even in molar acid. The same mode of η 1-N attachment also occurs on gold but saturation or space-limited coverages are not attained in very acidic media. Evidently, the driving force for protonation of the nitrogen heteroatom in molar acid is about the same magnitude as its coordination to the Au surface. The surface-coordination strength of DHPz is therefore greater on Pt than on Au. Chemisorbed DHPz can be hydrogenated and oxidized at both electrodes, although the extent of these reactions is lower on Au than on Pt. Coulometric measurements suggest that oxidation of surface-coordination DHPz is limited to cleavage of the NN bond and formation of terminal nitro groups.