The defects in the as-deposited and nitrided Si/SiO2/Hf1-xSiO(2) stacks have been analysed by EPR spectroscopy. The interface defects at the Si/SiO2 section are identical to those in the classical Si/SiO2 case and their concentration is not influenced by the hafnium silicate layer growth. As deposited hafnium silicate layers present in addition a defect located in the near surface region with a high concentration; it shows the characteristics of the EX center in SiO2 including low temperature hydrogen passivation. In the nitrided samples both the interface defects and the EX center are no longer observed. Additional annealing shows this to be related to a hydrogen passivation during the nitridation. The composition of the as deposited Hf1-xSixO2 layers have been analysed by ion beam analysis. Their thermal nitridation by NH3 has been investigated as a function of the annealing conditions. The hafnium silicate layers incoporate N up to some at% in the entire layer whereas hafnium oxide layers show no significant nitrogen incorporate into the bulk of the film.