The Pt-W(lOO) system has been studied using Auger spectroscopy. X-ray photoelectron spectroscopy and X-ray photoelectron diffraction. Several different regimes were discerned and characterized; a pseudomorphic first Pt layer, distorted hexagonal Pt overlayers, 3D Pt microcrystallites and alloy films. XPS indicated electron transfer from Pt to W at the W/Pt interface and in the alloy layers. Surface Pt inhibits the dissociative chemisorption of CO via an ensemble effect resulting in enhanced molecular adsorption. Distortion in the pseu-dohexagona! layer results in CO bonding to Pt not typical of Pt(l 11). On alloy films CO appears to bind only at W sites with the bonding modified by neighbouring Pt atoms (ligand effect). Surface Pt inhibits adsorption and dissociation of hydrogen on W. At submonolayer coverages there is evidence for occupation of mixed W-Pt sites. Pseudomorphic Pt shows H2 desorption behaviour not typical of pure Pt surfaces while thicker layers resemble Pt(lll). In the presence of large quantities of Pt, H2 desorption from W sites occurs in a new feature at 270K. In the presence of adsorbed CO, the H2 desorption features are shifted to lower temperature, while CO seems little affected by hydrogen. In some cases new H2 desorption features indicate some local mixing of adsorbed CO and hydrogen. Low coverages of methanol or formaldehyde on W(100) lead to CO and H2 as the only decomposition products while at higher coverages, complexes giving rise to desorption of H2, Co, CH4, H2CO and CH3OH are formed. Small quantities of surface Pt eliminate the formation of these complexes. Methane is also formed via a different intermediate which is observed for Pt coverages < 0.5ML. At higher Pt loadings, adsorption of methanol or formaldehyde on W is inhibited. Thick Pt layers behave like pure Pt surfaces towards decomposition of methanol or formaldehyde giving CO and H2 only as decomposition products. On alloy surfaces, there is evidence for the formation of an H/CO complex with methanol decomposition but not with formaldehyde.