The energy band gap, alignment with Si and the chemical bonding of 3-4 nm thick Hfx Si1-x O2 films with 0≤x≤1 were investigated as a function of composition. Nitrogen was introduced by N plasma incorporation into Hfx Si1-x O2 films with x=0.3, 0.5, and 0.7 grown on a SiO2/Si stack by metal-organic chemical vapor deposition. The structure of the dielectric films was characterized by high resolution transmission electron microscopy. X-ray photoelectron spectroscopy was used to determine the band gap, as well as the energy band alignment with Si and the chemical structure of the films. The amount of Si in the films and the incorporated N were found to influence the band gap and the band alignment with Si. The band gap was found to gradually decrease with the increase in Hf content, from a value of 8.9 eV (for pure SiO2) to a value of 5.3 eV (for pure HfO2). These changes were accompanied by a reduction of the valance band offset relative to the Si substrate, from a value of 4.8 eV (for pure SiO2) to a value of 1.5 eV (for pure HfO2). In addition, we have found that the presence of Hf-N bonds increases the conduction band offset from a value of 2.7 eV, which was obtained when only Hf-O bonds are present, to a value of 3.1 eV. The changes in the band structure and band alignment of Hf-silicate films are explained based on the chemical structure of the nitrided Hf-silicate films.