The outdiffusion of Be implanted into GaAs has been found to be identical after capless or capped (Si3N4 or SiO2) rapid thermal annealing (RTA) at 900-1000°C and to depend on the Be dose and its proximity to the surface. The outdiffusion is more pronounced when the Be implant is shallow (<0.1 μm) and/or the Be+ dose is high (>1×1015 cm-2). It is demonstrated that the Be outdiffusion is driven by the presence of a highly damaged surface layer. Auger results suggest the formation of a BeOx compound at the surface of a high-dose (1×1016 cm-2) Be-implanted sample that underwent capless RTA at 1000°C/1 s. It appears that BeOx formation occurs when the outdiffused Be interacts with the native Ga/As oxides during annealing. All the Be remaining in the GaAs after a >900°C/2 s RTA is electrically active.