This thesis investigates the suitability of magnesium oxide (MgO) and magnesium silicate as possible high-k dielectric materials on both silicon and indium phosphide (InP) surfaces. Given that the emphasis of this study was controlling the semiconductor/high-k interface formation, the principle experimental technique used in these studies was x-ray photoelectron spectroscopy (XPS), however, additional techniques such as atomic force microscopy (AFM) and electrical characterisation have also been employed. XPS studies have shown that exposure to ambient conditions results in the rapid formation of magnesium hydroxide and carbide species on the surface of MgO thin films. In order to prevent the detrimental affect of ambient exposure MgO/Si MOS devices were fabricated using an optimised metallisation procedure, called the FUSI process. Electrical characterisation of FUSI devices show low (< 1012 cm-2) interfacial state densities, and allowed the dielectric constant of MgO (~10) to be calculated. Transmission electron microscopy (TEM) and high resolution synchrotron based XPS have identified the presence of thin (< 1 nm) amorphous magnesium silicate layers at the MgO/Si interface. Studies show no evidence for the formation of SiO2 upon MgO deposition with Mg silicate forming preferentially following MgO deposition onto both H-terminated and oxidised Si surfaces, and also following high temperature annealing. The mechanism by which Si oxide surfaces can be converted into Mg silicate has also been investigated. It has been shown that room temperature deposition of metallic Mg results in the formation of Mg silicide which decomposes during UHV annealing at 300 °C. Subsequent annealing to 500 °C results in the complete conversion of SiO2 to Mg silicate. As Mg silicide growth has been shown to be a vital intermediate step to silicate formation, the growth and stability of Mg silicide thin films on Si has also been investigated. InP studies have focused on the preparation of oxide free, chemically stable surfaces prior to MgO deposition. The effectiveness of different chemical wet etch preparation procedures are compared to that of in-situ atomic hydrogen cleaning based on both the removal of oxide species and the chemical stability of the cleaned surface. The deposition of Si and Mg silicate interfacial control layers (ICL) onto InP has been shown to effectively passivate the InP surface, inhibiting the growth of In and P oxide species upon MgO deposition and subsequent 500 °C annealing.