The dynamics of the dissociative adsorption of hydrogen on the clean and hydrogen covered tungsten (100) surface have been studied under UHV conditions. Using supersonic molecular beams the mechanism for dissociative hydrogen adsorption was studied and found to proceed via both a direct and an indirect route. Due to the poor mass-match of the hydrogen/tungsten system, the high trapping probabilities could not be explained by simple phonon processes. Instead, the presence of a dynamical resonance has been proposed where trapping results from a reduction in the zero-point vibrational energy of the incident molecule and possible resonance with a quasibound state in a vibrationally excited dynamic well. This mechanism is supported by hard-cube analyses, temperature independent sticking observations and the incident angle dependence of the initial sticking coefficient. Dissociation is then believed to occur at surface defects. Enhanced sticking is observed at finite hydrogen coverages and is believed to be due to a surface reconstruction where the reconstructed surface effectively generates surface defects and thus increases the number of dissociation sites. A computer simulation of the proposed indirect channel was used to test the proposed model and provided good qualitative agreement with experiment. The fundamental steps of the Water-Gas Shift reaction (WGS) over Cu(110) have also been studied using supersonic molecular beams. The rate-determining steps of both the forward and reverse reactions were studied in an attempt to probe the activation barriers within a UHV environment. On the clean surface, water molecules were not found to dissociate at any beam energy (<780meV), whereas in the presence of preadsorbed oxygen the dissociation was facile. The proposed dissociative adsorption of carbon dioxide to CO and adsorbed oxygen was, similarly, investigated although no adsorbed species could be isolated and no dissociative adsorption was observed (<1.405eV). The high barriers to the direct dissociation processes lead us to suggest that the WGS reaction proceeds via an indirect mechanism.