Abstract: A compound semiconductor whose elements are differently volatile (e.g. GaAs, where As is more volatile than Ga) can be surface-passivated (a Ga layer overlying GaAs) by the steps of (a) anodizing a compound semiconductor to produce a layer thereon of native semiconductor oxide, (b) reducing the layer of oxide at elevated temperature and allowing the more volatile element of the reduced oxide to evaporate away, to leave predominantly the less volatile element overlying the compound semiconductor, (c) depositing an anodizable metal on the surface of the less volatile element, (d) re-anodizing the resulting structure until oxidation has proceeded to beyond the layer containing predominantly the less volatile element and from 10 A to 50 A into the compound semiconductor, and optionally at any suitable stage (e) applying an ohmic contact to the back surface of the compound semiconductor or selectively to the top surface as required.