Indiffusion profiles of sulfur in gallium arsenide were determined by secondary ion mass spectroscopy. In order to evaluate the shape of the profiles, a set of coupled reaction?diffusion equations was solved numerically. From the simulated nonequilibrium indiffusion profiles of sulfur, which diffuses into gallium arsenide via the kick-out mechanism, both the diffusion coefficient and the equilibrium concentration of arsenic self-interstitials were simultaneously determined. Transmission electron microscopy revealed that, due to an arsenic supersaturation, extrinsic dislocation loops have formed. The Fermi-level effect is more pronounced at lower diffusion temperatures and provides an additional driving force for the loop formation, agreeing well with the occurrence of larger faulted loops at a diffusion temperature of 950 ?C rather than at 1100 ?C. The complex behavior of the sulfur indiffusion can be quantitatively described by taking into account extended defects.