Ion migration in two fluorinated bioactive glasses of significantly different durability was modeled through molecular dynamics simulations. Whereas the very different biodegradation of these glasses cannot be explained on the basis of their structural features alone, the analysis of the diffusive data highlights a strong connection between the glass durability determined experimentally and the activation barriers for ion diffusion extracted by the simulations, clarifying the source of the different solubility and suggesting that “dynamical” descriptors of bioactivity could represent a key tool to predict the macroscopic behavior of a biomaterial, in some cases more effectively than with the current structural descriptors.