The redox-electrode reaction of [Co(phen)(3)](2+/3+) (phen= 1,10-phenanthroline) in 0.1 M potassium halide solutions was studied kinetically at a platinum (Ill) single-crystal electrode (M = mol dm(-3)). The formal rate constants were found to be dependent on the electrolyte anions and to decrease in the order F- > Cl- > Br-. Furthermore, formal rate constants for this reaction in 0.1 M KF solution were obtained at platinum (111), (100), and (110) single-crystal electrodes and those modified with I-, SCN-, and CN- adlayers. It was found that Miller indices had little effect on the heterogeneous electron-transfer rate. The iodide monolayer retarded the electron-transfer reaction, whereas the reaction was accelerated by the modification with cyanide and thiocyanate. Similar results were obtained for the redox-electrode reaction of [Co(bpy)(3)](2+/3+) (bpy = 2,2 ' -bipyridine) at the Pt(111) electrode and that modified with iodide and thiocyanate. These findings are discussed based on the electrical double layer effect and the theory of heterogeneous outer-sphere electron-transfer reactions reviewed by Weaver [M.J. Weaver, in: R.G. Compton, Comprehensive Chemical Kinetics, vol. 27, Elsevier, Amsterdam, 1987, Ch. 1].