Li-7 and H-1 magic-angle spinning (MAS) NMR spectra were recorded for Li(Ni, Co, Al) O-2 samples. The Li-7 MAS NMR signal of the layer on the pristine material is close to that of Li2CO3 in close contact with it. Its lineshape, due to dipolar interaction with electron spins of the material, is drastically different from that of pure Li2CO3. Upon hydration, this layer grows, while H-1 NMR suggests that some lithium in the material is replaced by protons. Upon electrochemical cycling, the original layer is replaced by a different one, the solid electrolyte interphase, with distinct Li-7 and H-1 NMR signatures, showing its largely organic nature. (C) 2004 The Electrochemical Society.