Summary: Measurements from powder diffraction patterns recorded at temperatures up to 950° C for the supposed equilibrium forms of albite synthesized at 800°, 900°, and 1000° C by MacKenzie (1957) indicate that only the form synthesized at 1000° C becomes monoclinic. Single-crystal measurements on heated pure natural albite (Ab_99·26) show that it becomes truly monoclinic, both in geometry and intensity distribution, at 930° C, the same temperature as for the highest-temperature synthetic form. These experiments were carried out in conditions that exclude possible contamination by potassium.