This work examines the suitability of both ultrathin Silicon Oxynitride (SiON) and Hafnium Silicate (HfSiON) layers to be used as high-permittivity gate dielectrics to serve as a replacement to silicon dioxide (Si0 2 )in future Metal-Oxide-Semiconductor (MOS) technologies. The reason Si0 2 needs to be replaced is the extremely high levels of leakage current displayed in ultrathin layers required for sub-90nm CMOS. The main part of the thesis consists of an electrical characterisation section, where the layers are evaluated in terms of their electrical reliability when fabricated into MOS devices, so as to determine their maximum operating voltage and performance during expected device lifetime. Techniques used include constant voltage stress, constant current stress, ramped voltage stress, charge pumping to determine interface state densities and stress induced leakage current measurements. Conventional methods of determining reliability are also evaluated. Results show that these methods cannot be blindly applied in ultrathin regime, and that finding an alternative dielectric material is a major challenge. The second part of the thesis consists of a chemical characterisation section, where blanket layers of the materials are examined using a range of surface analysis techniques. X-ray photoelectron spectroscopy (XPS), synchrotron based photoemission and secondary ion mass spectroscopy (SIMS) are used to determine the chemical composition and chemical depth profile information of the layers and to probe the electronic structure of the valence bands and allow the valence band offsets to be determined.