The diffusion of Sb in Si at concentrations around its solid solubility has been studied by isoconcentration experiments. The samples, grown by molecular-beam epitaxy, had constant Sb121background dopings and a Sb123spike embedded in this background. The diffusion was followed as a function of Sb background concentration at two different temperatures of 872 and 1019 °C by secondary ion mass spectrometry, differential Hall/resistivity measurements, and transmission electron microscopy. At concentrations exceeding the solid solubility Sb precipitates and interstitial-type dislocation loops were observed. At these concentrations the diffusivity decreased with increasing Sb background concentration. At concentrations below both the solid solubility and the intrinsic carrier concentration, for the highest diffusion temperature of 1019 °C, the diffusivity increases with increasing Sb background doping. This behavior is discussed considering mobile Sb2V complexes. ©1997 American Institute of Physics.