In this paper the deposition of very thin films of TiO2 on different substrates (SiO2, MgO, Ag) is studied by XPS. Shifts in the Ti 2p BE and the Auger parameter of Ti (alpha’ = Ti 2p BE + Ti L(3)M(23)V Auger KE) are observed on the three substrates. The magnitude and the sign of the shifts with respect to those of bulk TiO2 depend on coverage and on the type of substrate. In a parallel way, the magnitude of the energy gap of thin films of TiO2 changes depending on the substrate. This has been shown by UV-vis absorption spectroscopy for TiO2 deposited on SiO2 for a TiO2-Ag "cermet" (ceramic-metal thin film) and by photoemission with synchrotron radiation for TiO2 deposited on SiO2. It is proposed that the shift in the Auger parameter and the energy gap of TiO2 in these systems are two related parameters. Molecular orbital calculations (extended Huckel and INDO/1) with clusters simulating the TiO2-substrate interface explain qualitatively the variations in the Auger parameter and in the energy gap, The contribution of the polarization of the medium to the changes in the Auger parameter is approximated with an electrostatic model that accounts for the influence of the support in the observed shifts.