We show that the electrical properties of heavily boron-doped polycrystalline chemical vapor-deposited diamond films are modified by post-deposition thermal annealing with slow heating and cooling rates. We found that the boron effective activation energy increased after heating and cooling cycles in the temperature range of 300-673 K. The improvement also depends on the doping level. Samples with a resistivity of 6.41 m Omega.cm, 3.84 m Omega.cm and 0.826 m Omega.cm showed improvements of 6.4, 11.5 and 205%, respectively. Scanning tunneling microscopy (STM) images show that there were no exceptional changes in film morphology produced by the annealing processes, only an apparent cleaning of the surface at the nanometer scale. (C) 1998 Elsevier Science S.A.7912591262