Organic-inorganic composite macromolecular thin films have been grown by combined r.f.-magnetron sputtering of lithium orthophosphate (Li3PO4) and plasma polymerization of ethylene (C2H4). Of the process gases used, Ar, N-2, Ar/N-2, and Ar/O-2, only the Ar/O-2 mixture provided suitable control over the co-deposition process. The structure and electrical properties of the smooth, pinhole-free films were investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, Rutherford backscattering spectroscopy, X-ray diffraction, and a.c.-impedance measurement. The inorganic component of the composite films was found to exist both as a separate phase and as a part of the polymeric backbone. The ionic conductivity was found to decrease with the organic content of the film.