The dynamics of the interaction of electron‐cyclotron‐resonance (ECR) plasmas with the surface of GaAs substrates are studied by real‐time optical reflection spectroscopy. Analysis with a three‐phase ambient/overlayer/substrate model yields information on the time‐dependent composition of the near‐surface region, such as the thickness, degree of amorphization, and oxide and void fraction in the overlayer. Using this technique, it is observed that the thickness of the damaged layer formed by the impact of energetic ions increases linearly with the ion energy during argon ECR sputter etching. Furthermore, the dynamics of a cleaning process with a hydrogen ECR plasma have been studied. At temperatures between 300 and 500 °C this cleaning can be characterized by a two‐step process. During the first few seconds of exposure, the oxide layer is removed; in the second step, the GaAs is etched gently, which leads to a surface region with little damage to the crystal. At lower temperatures, cleaning is not successful and a thick damaged overlayer is formed. ©1995 American Institute of Physics.