A complete Ultra High Vacuum (U.H.V.) system has been developed to study the adsorption of gases on single crystal surfaces. Three surface sensitive techniques, namely Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Ion Scattering Spectroscopy (ISS), are included in the apparatus. The three techniques are available within the same experimental chamber. This allows the experimental techniques to be applied sequentially to the same surface. This experimental system has been used to study the adsorption of oxygen on Cu(100) and Ni(lOO) and ethylene on Ni(100). The ISS results obtained cannot be interpreted using elastic scattering arguements. The other major factor determining ion yields is neutralisation. Thus, a model of localised neutralisation has been developed. If the surface structure is taken as known, this model can be used to interpret successfully the ISS data from the Ni(100)(√2x√2)R45° -0 and Ni(100) (2x2)-C surfaces. The same model of localised neutralisation has been applied to the oxygen adsorption on Cu(100). The model does not allow unambiguous surface structure analysis to be performed. However, qualitative interpretation can be achieved by consideration of the differences between the ISS data obtained from the three surface systems. The best agreement with the experimental data is found for a Cu(100) )(√2x√2)R45° -0 surface with a combination of two fold and four fold adsorbed oxygen atoms. However, the level of agreement is not sufficient for more than tentative acceptance of this structure. The major implication of this work for the use of ISS in surface structure analysis is that localised neutralisation effects dominate for the (100) surface. Previous surface structural studies using ISS have been successful on the (110) crystal face, where the 'ridged' surface makes structural interpretation less sensitive to the shadowing model. Thus, for a complete understanding of the factors determining the relative ion yields, both ion neutralisation and elastic scattering must be considered.