Anodisation of a germanium surface was carried out successfully in oxygen plasma excited by a high-frequency (500 kHz) electromagnetic field in a quartz reactor. The refractive index of the oxygen plasma anodised (OPA) film on Ge(111) is generally 1.66-1.68 ( lambda =6328 AA). The chemical composition and chemical states of the Ge-OPA film have been analysed using Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) with argon ion sputtering. There is GeO₂ on the surface of the OPA film. In the bulk of the OPA film, there is mainly GeO₂ and some unoxidised germanium. The distribution of O and Ge is fundamentally homogeneous in the bulk. There are double peaks for Ge(3d) in the XPS of the Ge-OPA film after argon ion sputtering. The binding energy of 32.7 eV is due to the Ge in GeO₂, and that of 29.2 eV corresponds to a few unoxidised Ge atoms in the OPA film. The chemical stoichiometry of the OPA film is very similar to that of vitreous GeO₂.