Effects of annealing in hydrogen ambient (sintering) of oxide-nitride-oxide (ONO) structures grown onto p-doped silicon substrates have been studied by X-ray photoelectron spectroscopy, time of flight-secondary ion mass spectroscopy, and electrical current-vs.-voltage techniques. The behavior of the Auger parameter determined during the depth profile and time of flight-secondary ion mass spectroscopy results show how the sintering process changes the stoichiometry of the bottom layer of silicon oxide grown onto the silicon substrate by saturating the dangling bonds present in the unannealed structure and how a greater oxidation time to form the top oxide layer, producing a better quality oxide, improves the electrical performances of the whole ONO structure.