We present a comparative study of the electrical and structural characteristics of metal–insulator–semiconductor (MIS) devices using SiN1.55:H or SiN1.55:H/SiOx stacks as gate dielectrics, with the aim of improving the thermal stability of the SiN1.55:H/Si interface. The dielectrics were grown on Si by the electron cyclotron resonance plasma method. The stacks were produced by plasma oxidation of the Si surface, resulting in a thin layer of SiOx (PO–SiOx), followed by deposition of the SiN1.55:H layer. Afterwards, the samples were rapid thermally annealed (RTA) at temperatures ranging from 300 to 1000 °C. Some representative samples were studied by Fourier transform infrared spectroscopy to characterize the bonding structure of the SiN1.55:H/PO–SiOx stack and its evolution with the annealing temperature. The results were in good agreement with the well known model for suboxide evolution with the temperature: the formation of highly defective nanocrystalline Si clusters inside a high quality SiO2 matrix. This process takes place for RTA temperatures higher than 700 °C. This model also explains the results derived from C–V measurements, which show improvement of the PO–SiOx/Si interface as a result of the formation of the high quality SiO2 matrix. Additionally, the thermal stability improved with respect to the SiN1.55:H/Si interface, which suffers significant degradation when annealed above 600 °C, while devices made with the stacks are able to hold annealing temperatures up to 900 °C. On the other hand, the formation of nc-Si results in degradation of the reliability of the MIS devices.