Diffusion in a deformable medium shows a remarkable transition at the onset of concerted particle-medium motions. This is derived for Brownian motion in a periodic potential coupled to a broad phonon spectrum, and observed for rotation of organic molecules inside zeolite cavities. Interactions between the particle and local overdamped phonons in the potential wells cause residence-time dependent lattice deformations, which, in our model, decrease the energy barriers. The model explains the non-Arrhenius type of rotational motion of pentane inside zeolite ZK-5 observed with H-1-C-13 NMR relaxometry. (C) 2003 Elsevier Science B.V. All rights reserved.