Compositions of Li-Ge-O electrolyte thin films (0.1 to 3 mu m thick) deposited by r.f. sputtering were determined by Rutherford backscattering spectrometry (RBS) and proton induced gamma-ray emission spectroscopy (PIGE). The aim of this paper is to illustrate the interest of these techniques in composition determination of thin films. The use of these techniques allows the optimization of Li-Ge-O thin films composition giving the highest ionic conductivity.