On the basis of an X-ray study of the thermal deformation of solid solutions with an ordered beta -eucryptite structure, it is shown that during heating some of the lithium atoms are transferred from 4-coordinated to 6-coordinated positions. This causes compression of the structure along the c-axis and expansion of it along the a-axis. In the case of the crystalline phase of the glass ceramic a large portion of the lithium atoms is already in 6-coordinated positions, so that the thermal expansion of this phase is not affected by the disordering.