The magnetic properties of thin (Ga,Mn) As layers improve during annealing by out-diffusion of interstitialMn ions to a free surface. Out-diffused Mn atoms participate in the growth of aMn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn) As layer.