We have investigated advanced MOS structures containing Ru gate electrode, HfxSi1-xOy dielectric film and Si substrate. The Ru gate electrode was grown by MOCVD at 300 °C. The MOS structures were annealed for 30 min in forming gas and nitrogen at temperatures up to 550 °C. Capacitance-voltage measurements showed important shift of the flat band voltage of the Ru/ HfxSi1-xOy/Si gate stack after treatment at 550 °C. X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), reflection electron energy loss spectroscopy (REELS) and secondary ion mass spectroscopy (SIMS) were used to analyze interface between ruthenium and high-k dielectric film. Based on the analysis we were able to build up energy-band alignement for the Ru/ HfxSi1-xOy interface. We observed that the energy-band structure of the Ru/HfxSi1-xOy interface remains stable upon annealing in forming gas up to 550 °C. Presence of hydrogen revealed by SIMS can account for compensation of negative charges in HfxSi1-xOy during thermal treatment.