The electrochemistry of xanthine oxidase has been examined at glassy carbon and mercury electrodes. No response is observed at glassy carbon unless denaturation of the enzyme has occurred in which case a surface-confined chemically reversible process FAD + 2 H+ + 2 e- ⇄ FADH2 is observed (FAD = flavin adenine dinucleotide). In contrast, at mercury electrodes, processes associated with FAD and with enzyme-bound mercuric cysteinate, cysteine and cystine groups are observed, even under conditions where no free FAD is detected at a glassy carbon electrode. Study of these processes as a function of pH, after interaction with Hg2+ and after conversion of xanthine oxidase to xanthine dehydrogenase, allows a self-consistent reaction scheme to be proposed. Interestingly, at the mercury electrode, no potential is available at which a redox process does not occur and consequently, denaturation of the protein takes place at the electrode surface at all potentials. No molybdenum- or Fe2S2-based electrochemistry was detected.