UHV and ambient Scanning Tunnelling Microscopy (STM) have been used to investigate the interaction of sulphur with Si and GaAs (100) and (111) surfaces. The adsoiption of group VI elements on GaAs and other IE-V semiconductor surfaces is well known to passivate the surface, that is, reduce the number of mid-gap surface states. We find that in situ room temperature adsorption of sulphur on both Si(100)-(2xl) and S i(lll)-(7 x 7 ) surfaces, using an electrochemical cell, does not produce an ideal bulk terminated ( lx l ) phase. Thermal desorption of the S overlayer from the Si(100) surface at 325°C leads to the creation of a c(4x4) reconstruction coexisting with the (2x1) reconstruction of the clean surface. High resolution filled- and empty-state images have led to the proposal of a missing dimer defect model for this reconstruction. Following sulfur desorption, at 375°C, from the S i ( l l l ) surface, monolayer deep holes in the (7x7) terraces are clearly visible. Near the edges of these holes, surface atoms are found in either a disordered phase, or forming small areas of other metastable reconstructions. A coalescence of surface vacancies, following defect creation due to sulfur desorption leads to the monolayer etching mechanism. Use of As capped GaAs samples made a characterisation of the clean surface, before sulfur deposition, possible. Room temperature adsorption of sulphur on both GaAs(lOO) and GaAs(l 11)B surfaces leads to the appearance of a ( lx l ) LEED pattern.We find no evidence from STM imaging of a well ordered surface corresponding to this ( lx l) phase. Instead, the ( lx l ) LEED pattern arises through an amorphous S overlayer saturating dangling bonds. Annealing of the S covered GaAs(lOO) surfaces to temperatures above 350°C promotes the formation of a (2x1) reconstruction. STM data, coupled with Auger electron and synchrotron radiation photoelectron core-level spectroscopy studies, suggest that the (2x1) surface most likely consists of both As and S dimers with sulfur also diffusing into the bulk GaAs crystal. For the G aA s ( lll)B surface, after annealing, a similar situation involving both As and S termination, occurs. Fermi level movement due to sulfur adsorption and subsequent annealing on both (100) and ( l l l )B surfaces is discussed. Ambient STM and tunnelling spectroscopy measurements on P2S5/(NH4)2SX treated GaAs(lOO) samples indicate that, with regard to surface homogeneity, they are topographically, chemically and electrically much superior to etched, untreated samples. The subsequent oxidation of these surfaces has been investigated using the real time 3- dimensional imaging capabilities of the STM. Tunnelling spectroscopy results show a considerable reduction in band bending for the passivated surface, with evidence to suggest that the Fermi level is unpinned.