We demonstrate that sulfur in-diffusion is a suitable tool to study the diffusion on the arsenic sublattice. The interaction of sulfur with dislocations and the formation of extended defects is investigated in detail by means of cathodoluminescence and transmis- sion/analytical electron microscopy. The sulfur in-diffusion is governed by the kick-out mechanism. The diffusion proceeds for low sulfur concentrations under equilibrium conditions for As self-interstitials. In this case the diffusion profile is errorfunction-like and essentially determined by the sulfur diffusion coefficient. For higher sulfur concentration the sulfur diffuses under non-equilibrium conditions. Extended defects were formed during the in-diffusion process. They were observed up to a depth larger than the sulfur diffusion profile. The diffusion into dislocation-rich GaAs was investigated to study the interaction of sulfur with extended defects. An enrichment of sulfur at dislocations was found.