We studied the strain induced in a silicon lattice by the presence of boron clusters formed by the interaction of high concentration (2× 10 20 at cm− 3) substitutional B with Si interstitials produced by ion implantation. We found that the clustered immobile B induces a significant amount of strain in the host lattice. By analysing the dissolution behaviour of boron clusters after prolonged annealing, we found that the strain linearly decreases as the amount of clustered B decreases, evidencing a constant lattice expansion ΔV per clustered B atom of (3.7±0.6) Å 3, independently of the annealing temperature and time. By performing model potential calculations of ΔV on selected boron clusters, we concluded that these clusters can be structurally more complex than those commonly assumed or hypothesized so far.