Perovskite oxide membranes (La0.6Sr0.4Co0.2Fe0.8O3) are used for the separation of oxygen from air. In oxygen permeation experiments these membranes showed a peculiar behavior. Besides poor performance, a characteristic coloring also appeared on the surface of the membranes. In order to understand what was happening to the surface of the membranes, they were analysed with Low-Energy Ion Spectroscopy (LEIS) and X-ray Photoelectron Spectroscopy (XPS). The analyses showed that the surface of the LSCF membrane was covered with a SiO2 layer, which obviously reduced the performance and caused the coloring. It was established that the source of the silicon was siloxane containing grease that was used in the manual valves of the setup. In a new improved permeation setup, where grease-free valves were used, the LSCF membranes showed remarkably better performance. The LEIS measurements showed also that the permeation experiment of 300 h did not affect the surface composition of the membranes. The contamination-free LSCF membranes only showed the presence of La, Sr and O in the outermost atomic layer. The observed absence of Co and Fe suggests that further improvement of the membrane performance is possible.