The same experimental techniques as used earlier to characterize the composition and properties of the so-called solid electrolyte interphase (SEI) layer formed at the graphite-anode–electrolyte interface of a Li-ion battery are used here to acquire some degree of understanding of interface phenomena occurring on the cathode side of the cell, even though the validity of the SEI-layer concept is still somewhat tenuous in this “cathode” context. We here probe cathode-related SEI phenomena for the three cases: LiMn2O4, LiCoO2/LiNi0.8Co0.2O2, and carbon-coated LiFePO4. The various layer types formed have been analyzed systematically for different salts, solvents, cycling modes, storage times, temperatures, etc., using photoelectron spectroscopy (PES). Depth-profiling of the layers formed was achieved using Al Kα radiation in conjunction with Ar-ion sputtering; non-destructive depth-profiling was made possible using synchrotron radiation, and applied to the important case of carbon-coated LiFePO4. A number of trends have emerged from our studies, and some general models are proposed to reflect features characteristic of the various systems studied. Our results are related to the more familiar SEI-layer formed on graphite.