A study is made of the properties of a paramagnetic ion in an E_g orbital state interacting with a lattice. A general formulation is given, appropriate to any spin, and it is shown that, if the ion-lattice interaction is sufficiently great, for the low-lying levels a profound change occurs in the motion of the neighbours. This may properly be described as a dynamic Jahn-Teller effect. The nature of the motion is examined and it is suggested that the ion plus its neighbours acts as a non-linear scattering centre for incident thermal waves, and that there are local vibrations of comparatively large amplitudes at `beat frequencies'. In any situation where this description is applicable it then appears to be a reasonable approximation to replace the ion plus lattice by a `molecular model'.