Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl-2, CF4, Ar and CH4, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min Of Cl-2 and CF4 plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)(6)(3-/4-) and Fe2+/3+ at the BDD electrodes was found to decrease after Cl-2 and CF4 plasma treatments. However, the electron-transfer rate for Ru(NH3)(6)(2+/3+) showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH4 plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize, diamond surfaces in various ways while maintaining a wide potential window and a low background current. (c) 2006 Elsevier Ltd. All rights reserved.