The high temperature adsorption of oxygen and the room temperature adsorption of CO on the Pt3Sn(111) and Pt3Sn(110) surfaces have been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Beforehand the structure of the clean surfaces has been reviewed. After exposure to several 1000 L O2 at sample temperatures of about 750 K on both Pt3Sn(111) and (110) an ultra-thin Sn-O surface layer is formed. For the (111) X-ray photoelectron spectroscopy (XPS) indicates that this layer does not yet exhibit oxide properties. STM topographs of the Sn-O phase show on both surfaces meshes of highly corrugated protrusions commensurate with the substrate. In the case of the (111), after additional thermal annealing with STM and LEED a (4 × 4) reconstruction is observed, that is due to a (2 × 2) supermesh of depressions in the p(2 × 2) mesh of protrusions. This structure is similar to findings reported for the oxidation of Sn/Pt(111) surface alloys. X-ray photoelectron diffraction (XPD) measurements in comparison with simulations yield a tentative model for the (111) Sn-O layer. On the Pt3Sn(110) surface after oxygen exposure a c(2 × 2) hexagonal grid of protrusions with regard to the (2 × 1) substrate is observed with STM and LEED. STM reveals the existence of domains due to two equivalent positions of the Sn-O layer relative to the substrate. The domain boundaries zigzag around the [1-10] direction. The Sn-O layer can on both surfaces be removed by thermal annealing to more than 1050 K. After CO adsorption at room temperature on both Pt3Sn(111) and (110) adsorbate structures are observable with the STM. On the (111) two different types of structures are found: ordered patches of protrusions and unordered clusters. These structures are seen only on (√3 × √3)R30° substrate regions, not on p(2 × 2) regions. Surprisingly on the (110) the CO molecules mostly arrange in dimers. For both (111) and (110) saturation coverage is already reached at about 30% of a closed monolayer. The CO can be desorbed by slightly heating the samples to about 400 K. STM topographs show that on both surfaces CO adsorbes in Pt sites, not on Sn. It was possible to observe the CO adsorption on the (110) directly live with the STM. The observed adsorption processes hint to a dimer formation mechanism where a preadsorbed monomer and a CO molecule form the gas phase or a precursor phase stick together. When on partially Sn-O phase covered Pt3Sn(111) and (110) surfaces CO is adsorbed at room temperature, the respective structures coexist. Neither is CO observed on the Sn-O phase nor does a reaction between CO and O occur.