A monocrystalline compound semiconductor substrate (1) is passivated with a layer (3) of the most volatile element of the semiconductor compound to prevent the formation of oxides that would interfere with further processing. A surface layer of arsenic is formed on a GaAs substrate by exposing the substrate to light having a photon energy greater than 1.8 eV, at a power density of 0.01 to 0.5 watts per cm 2 for a period of 10 to 30 minutes while the substrate is immersed in a 1 : 1 HCI : H 2 0 solution. The passivated substrate may be stored and handled in air. When desired, the As layer can be removed by low temperature baking, for example at 150° to 300°C.