We show that the electrical properties of nitrogen-doped nominally undoped polycrystalline chemical vapor deposited diamond films are modified by post-deposition heating in an oxidizing atmosphere. We found that the first heating cycle in air in the temperature range of 300–673 K decreased the graphitization content still present in the diamond surface and that after the second heating cycle the electrical resistance versus temperature curves became stabilized. Using a flow of argon with residues of oxygen over the surface of the sample during the heating cycles, the stabilization of the resistance-temperature dependence also occurred but only after the fourth heating cycle. The results suggest the existence of an oxidation mechanism of the nondiamond carbon atoms present at the diamond surface. After stabilization, the deep donor ionization energy was found to beEd=1.62±0.02 eV.All results brought together strongly suggest that this level is due to single nitrogen atoms that occupy substitutional lattice sites in diamond. ©1998 American Institute of Physics.