This article describes a study of surface changes of (100) GaAs subjected to rf hydrogen plasma treatment. Hydrogen plasma etching of GaAs leads to the reduction of the As oxide in the native oxide film, leaving a wholly Ga oxide surface film. Further etching of the surface leads to the production of a nonstoichiometric subsurface amorphous GaAs layer which varies from Ga rich at low chamber pressures to As rich as the pressure is increased. This effect is dependent on the ion energy and may be explained in terms of the penetration depth of the hydrogen ions relative to the oxide thickness.