Vacuum-annealing imparts conductivity to initially insulating undoped polycrystalline chemical-vapor-deposited diamond, thus turning it to a possible electrode material. The diamond film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10(11) to 10(12) Omega cm down to less than 0.1 Omega cm; the differential capacitance increased from similar to10(-3) to -50 muF per 1 cm(2) of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN)(6)](3-/4-) redox solution increased from similar to0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes in the electrode properties are attributed to gradual change in the thickness and/or properties (first and foremost, conductivity) of the nondiamond carbon phase formed along the intercrystallite boundaries upon the annealing; the conducting phase is outcropping at the film surface as an array of microelectrodes ("active sites"). (C) 2003 Elsevier Ltd. All rights reserved.