The variation of the birefringence with temperature, from room temperature up to c. 900° C, was studied of 34 nephelines from different geological environments. The measurements were made with a Berek compensator using a microscope heating stage. Two optical types of nepheline were distinguished on the basis of the absolute value for birefringence at room temperature and on the basis of the optical Δ-value defined as >=(w-E)900°-(w-E)20°. The optical type I is characterized by a high birefringence at room temperature and by a highly negative optical Δ-value, whereas the optical type II shows a low birefringence at room temperature and a highly positive optical Δ-value. The nepheline specimens studied represent optically a continuous gradation from type I to type IL Both optical types are represented among mediopotassic and perpotassic nephelines. A nepheline of the optical type I is considered to represent the ordered form of the mineral and a nepheline of the optical type II the disordered form. All plutonic nephelines studied belong to the optical type I whereas the volcanic nephelines range from the type I to the type II. Using a high temperature attachment for the Norelco diffractometer, the thermal expansion was measured of a few nepheline specimens. A nepheline belonging to the optical type I differs slightly in thermal expansion from that of the optical type 11. This difference in thermal expansion may qualitatively explain the difference between the two optical types. The crystal structural nature of the supposed order-disorder transition in nepheline is not known. The analogy with the feldspars would suggest a varying degree of ordering in the distribution of the silicon and aluminium cations in the oxygen tetrahedra.