We have studied the electrochemical impedance spectroscopy of conductive ultrananocrystalline diamond (UNCD) modified by either oxidation or hydrogenation surface treatments. The impedance was measured in the frequency range from 0.1 Hz to 40 kHz at different DC voltages and the results fitted to an equivalent electrical circuit. Despite the complexity of the conductive UNCD surface, composed of sp(3)-bonded grains and grain boundaries with a high content of sp(2)-bonded carbon atoms, a Randles circuit with a constant phase element (CPE) for the capacitive element provided a reasonable model for both terminations. However, the parameters of the CPE were very different for each termination. Taking into account the results obtained, we propose that the interfacial impedance of oxidized UNCD is dominated by the oxidized sp(2)-bonded Carbon atoms present at the grain boundaries, and the interfacial impedance of hydrogen-terminated UNCD is governed by both the grain boundaries and the grains. (C) 2008 Elsevier Ltd. All rights reserved.