Abstract Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl 2, CF 4, Ar and CH 4, 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 CF 4 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 Fe 2+/3+ at the BDD electrodes was found to decrease after Cl 2 and CF 4 plasma treatments. However, the electron-transfer rate for Ru(NH 3) 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 CH 4 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.