We study the size dependence of the Mössbauer recoilless fraction in β-Sn nanocrystals by performing conversion electron Mössbauer spectroscopy at different temperatures. The Mössbauer recoilless fraction is intimately related to dynamical processes involving the nuclei during the γ-ray emission. The β-Sn nanocrystals are embedded in a SiO 2 matrix, and they have a mean diameter ranging from 7 to 17 nm. A lowering of the recoilless fraction with decreasing cluster size is observed. The smallest nanocrystals reveal a 60% reduction of the recoilless fraction compared to the bulk value, while for the largest clusters we observe the same Mössbauer recoilless fraction as in the bulk. This suggests that the dynamical properties of the β-Sn nanocrystals in SiO 2 approach those of the bulk for a critical mean diameter above 16 nm. The experimental results are compared with theoretical values obtained by a continuum …