Electrochemical properties of nanoporous honeycomb diamond electrodes in an aqueous electrolyte were investigated. Highly ordered honeycomb diamond structures were fabricated by etching the microwave plasma-deposited diamond films using an oxygen plasma. The diamond honeycomb exhibited a wide electrochemical potential window (ca. 2.5 V), similar to the unetched diamond film. From impedance measurements, the capacitance of the diamond honeycomb film was found to be 1.97 X 10(-3) F cm(-2) (geometric area), which is cn. 200 times greater than that for the unetched, as-deposited surface. The results obtained with galvanostatic measurements were consistent with this value. The formation of the highly ordered porous structure, together with surface oxidation, was found to be responsible for the observed enhancement in the capacitance. The transmission line model for cylindrically porous electrodes was successfully applied to the present honeycomb structure. Based on an estimation of the capacitance of a hypothetical through-hole diamond membrane, the specific capacitance is ca. 16 F g(-1), which is within an order of magnitude of the range for activated carbon capacitors (100 to 400 F g(-1)).