Changes of electronic structure during nitrogen implantation into heated Ti thin films, which were accompanied by crystallographic structural transformation, were studied by applying in situ electron energy loss spectroscopy (EELS) combined with self-consistent charge discrete variational (DV)-Xalpha molecular orbital (MO) calculations. Hydrogen atoms constituting TiHx were released from the films with heating at temperatures more than about 100 degreesC. The H-released unstable fcc-Ti was transformed into hcp-Ti. The energy loss peak observed by EELS for TiKx shifted to lower energies with the release of H atoms during heating. Nitrogen ions of N-2(+) with 62 keV were implanted into the hcp-Ti films held at 350 degreesC. The TiNy films were "epitaxially" formed by the transformation of hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral sites of the fec-Ti sublattice by N atoms. The energy loss peak for hcp-Ti films observed by EELS during N implantation gradually shifted to higher energies with increasing the dose. The shift meant increase in the density of electrons occupying the hybridized valence band. The formation mechanism of TiNy was discussed with the results of the calculated overlap populations of Ti-Ti and Ti-N bonds. (C) 2004 Elsevier B.V. All rights reserved.