A transition in which the degree of order of the system changes. Three principal types of disordering transitions may be distinguished: (i) positional disordering in a solid, (ii) orientational disordering which may be static or dynamic and (iii) disordering associated with electronic and nuclear spin states. Examples:

1. The transition of LiFeO₂, with a tetragonal unit cell, in which the Li⁺ and Fe³⁺ cations are perfectly ordered on crystallographically non-equivalent octahedral sites to cubic LiFeO₂ in which the Li⁺ and Fe³⁺ cations are distributed randomly over all the octahedral sites.
2. The transition of orthorhombic KCN to cubic KCN in which the CN⁻ ions become oriented in any of the eight [111] directions.
3. A superconducting transition