Positron annihilation spectroscopy was applied to Si-doped GaAs grown by molecular-beam epitaxy. Annealing SiO2 capped samples at 930C caused the SiGa - VGa vacancies to become positively charged. The SiO2 caps cause O to diffuse beneath the surface and the spectroscopy led to the first measurement of the O diffusion coefficient in GaAs. The application of a new analysis program ROYPROF reveals the creation of internal electric fieldsPositron Annihilation Study of the Diffusion of Oxygen in Annealed Silicon-Doped Gallium Arsenide. A.C.Towner, M.Nathwani, A.S.Saleh, D.P.van der Werf, P.Rice-Evans: Philosophical Magazine B, 2002, 82[17], 1809-15