This Letter presents a new model to account for the mechanism of enrichment of Sb on cavity surfaces grown in heat-treated zones of low-alloyed ferritic steels. The model, based on the energetic balance between the change of the atomic free energy of an impurity atom that leaves a cavity inner surface and the elastic energy needed to accommodate the impurity atom in the crystal lattice, accounts well for the experimental Sb enrichment factors. The enrichment factors were measured as a function of the cavity radii by means a high-resolution Auger microprobe. The grain-boundary cavity surfaces were obtained, by fracturing in ultra-high vacuum a 1/2Cr1/2Mo1/4V steel sample, heat-treated at 800K for 9 × 104h.