The bare surfaces of rare earth metal systems were described above with respect to their structural and electronic behavior. As already mentioned occasionally in the previous chapter, adsorbates, e.g. from the residual gas, alter the electronic properties of the clean surfaces due to the hybridization of adsorbate atom and substrate states (for example, the Gd surface state can only exist if the surface is atomically clean). The strong influence of adsorbate atoms on the surface due to this chemisorption process does not only induce variations in the electronic properties—this will be discussed in Sects. 4.1 and 4.4 for hydrogen and oxygen being adsorbed on Gd(0001), respectively—but may also lead to distinct structural changes (e.g., surface reconstructions) which will exemplarily be shown in Sect. 4.2 for hydrogen being dissolved in gadolinium thin films. Using the distinct surface sensitivity of both techniques—scanning tunneling microscopy (STM) and photoelectron spectroscopy—it will be shown in Sect. 4.3, by describing the coadsorption of hydrogen and CO on gadolinium surfaces, that their combination allows a significantly more detailed analysis.