The interaction of reactive and unreactive metals with clean, oxidised and sulphur passivated GaAs(lOO) surfaces has been investigated using a range of experimental techniques. The deposition of both gold and iron on chemically cleaned and oxidised GaAs(lOO) surfaces has revealed a correlation between the magnitude of the ideality parameter determined by current-voltage measurements and the presence of interface states as detected by the deep level transient spectroscopy (DLTS) technique. Schottky barrier diodes exhibiting near-ideal behaviour were fabricated by deposition of both gold and iron on GaAs surfaces immediately after chemical etch in H2S 0 4:H20 2:H20 (5:1:1). No interface states were detected on these diodes. However, metals deposited on oxidised surfaces gave rise to highly non-ideal IV characteristics. In addition, DLTS studies revealed the presence of interface states, caused by defects induced during the oxidation of the GaAs surfaces prior to metal deposition. The passivating effects of sulphur in the forms of hydrogen sulphide. H2S and molecular sulphur, S2 on the GaAs surface have also been investigated. In particular, the adsorption of molecular sulphur on the As decapped GaAs surface has been studied using surface sensitive synchrotron radiation. Clean GaAs (100) surfaces with different As to Ga surface atom ratios were prepared by the thermal decapping of a protective As overlayer deposited on MBE grown GaAs. Studies have shown that the uptake of sulphur by the As and Ga surface atoms depends strongly on the As to Ga ratio of the clean decapped surface. Such sulphur treatments result in the adsorption of a monolayer of sulphur w hich is bonded to both the Ga and As surface atoms. As a complement to sulphur passivation, which has the effect of slowing down and/or preventing oxidation of GaAs surfaces, the reduction and removal of oxides by the deposition of ultrathin layers of titanium has been investigated. By exposing clean GaAs surfaces to air at atmosphere, a monolayer of oxides was grown. The subsequent deposition of titanium resulted in the complete removal of the As and Ga oxides and the formation of an extremely abrupt Ti-TiOx-GaAs (x~l) interface.