Isothermal desorption of oxide layers formed on GaAs(001) surfaces of arsenic-rich c(4 4) reconstructions was observed at about 540°C using a quadrupole mass spectrometer. Ga 2 O and molecular arsenic desorbed in parallel. After long induction periods of the order of 10 min, their desorption rates increased exponentially, and then decreased steeply due to the completion of oxide removal. The behaviors observed in the final stage of oxide desorption suggest that the removal of the oxide layer proceeds through formation of voids, and that the oxide desorption rate is proportional to the area of the voids.