Oxygen-plasma treatment of tin-doped indium oxide (ITO) films electrodes was found to affect the electron-transfer kinetics of tris(2,28-bipyridyl) ruthenium(II) chloride [Ru(bpy)(3)(2+/3+)] and ferrocyanide [Fe(CN)(6)(4-/3-)], especially when fast scan rates were used for interrogation. Plasma treatment also reduced the density of hydroxyl groups at the ITO surface, as monitored through binding of 12-phosphonododecanoic acid to the ITO surface. Under the conditions used here, plasma treatment did not affect the surface morphology, microstructure, or elemental composition (for exposure times shorter than 10 min) of the film. We speculate that oxygen plasma treatment reduces the number of active electron-transfer sites at the electrode surface, possibly oxygen vacancies, resulting in slower electron-transfer kinetics. These findings suggest that care needs to be taken if oxygen plasma is used for microfabrication of small ITO electrodes, as even mild treatment can negatively impact electron-transfer kinetics at the ITO surface. (C) 2003 The Electrochemical Society.