The dc cyclic voltammetry of several redox proteins at thin film ruthenium dioxide electrodes is described. Quasi-reversible voltammetric responses were obtained for cytochrome c, azurin, ferredoxin, rubredoxin and plastocyanin at this electrode with solution conditions, pH 8.0 (5 m M Tricine) and 100 m M NaCl as supporting electrolyte. The values of E 1/2 are in good agreement with those in the literature. On varying the pH and solution ionic strength striking differences in response emerge. For the negatively charged proteins, ferredoxin, rubredoxin and plastocyanin, no measurable faradaic response is observed at lower ionic strength (pH 8.0, 1 m M N2Cl). The electrochemistry can, however, be promoted by multivalent cations (Cr(NH 3) 3+ 6 or Mg 2+) giving rise to a well-defined quasi-reversible response. For cytochrome c, variation of the dc cyclic voltammetric response with pH correlates with the (acid-base) protonation properties of the electrode surface. The electrochemistry of azurin does not vary appreciably with changes of pH or ionic strength. The results found highlight the importance of electrostatics at the electrode/solution interface. Preliminary results using IrO 2, Na 0.7WO 3 and single-crystal RuO 2 electrodes are reported. Subtle differences in behaviour are found at the single-crystal and thin-film RuO 2 electrodes.