In my thesis, I present an investigation of the dissociation reactions of gas phase molecules on single crystal metal surfaces studied by a molecular beam in combination with Reflection Absorption Infrared Spectroscopy (RAIRS). Two gas/surface systems were investigated namely water (D2O) dissociation on a flat Cu(111) surface and methane dissociation on a stepped Pt(211) surface. The dissociation of water molecules on an oxygen covered Cu(111) surface is presented first. In this study, I explain how I confirmed the reaction mechanism which leads to facile water dissociation on a Cu(111) surface pre-covered with oxygen atoms using isotopic labeling of the oxygen atoms coupled with RAIRS detection. Direct observation of the two different hydroxyl isotopologues on the surface showed that the water molecules dissociate on the surface by transferring an H-atom onto the oxygen adsorbate. At a surface temperature of 180 K, the reverse disproportionation reaction was observed to occur simultaneously with the dissociative adsorption of water molecules which resulted in the removal on the pre-covered oxygen isotope on the surface with continuous surface exposure to water molecules. The study of water dissociation is continued but for a clean Cu(111) surface i.e. no pre-adsorbed oxygen atoms. The sticking probabilities of the dissociated water molecules were investigated by controlling the translational energy and the vibrational energy introduced into the incident water molecules. This experiment showed that the translational energy is better in promoting the dissociative chemisorption of D2O on Cu(111) surface than vibrational energy. The interaction between methylidyne-CH(ads) co-adsorbed with H(ads) atoms on a Pt(211) surface was investigated. Three distinct RAIRS absorption peaks were observed for the CH(ads) on the surface where the intensity of the peaks could be transferred from one to the other by changing the H atoms coverage on the surface. These peaks were then assigned to the methylidyne with 0, 1 or 2 neighbouring hydrogen atoms. The three hydrogen environments arose as a result of the limited one-dimensional diffusion of hydrogen atoms on the steps of the Pt(211) surface. I conclude by summarizing the results from the studies presented in this thesis and showing future experiments that one could perform to understand the water molecules reaction on Cu surfaces better.