The dehydration of the natural zeolite thomsonite, Na4CasAl20Si20O80 24H(2)O, has been studied using a combination of conventional and synchrotron X-ray powder diffraction. A preliminary in situ dehydration study revealed two distinct unit cell changes at approximately 540 and 570 K, respectively. Thomsonite samples were heated to 523, 553, 583 K for one hour and then sealed in capillaries before data collection. Fully hydrated thomsonite: Spacegroup Pncn, a = 13.0814(2), b = 13.0429(2), c = 13.2004(2) Angstrom, and 24 water/unit cell. Two mixed CaA/Na sites share four waters, while the CaB sites coordinate two waters each. Thomsonite, heat treated at 523 K: Spacegroup Pncn, a = 13.0032(1), b = 12.9814(1), c = 13.2486(1) Angstrom, and 16 water/unit cell. Up to the first phase transition W3 and W4, both coordinated by the mixed CaA/Na site, are expelled. Thomsonite, heat treated at 553 K-Spacegroup Pn2n, a = 13.0814(2), b = 13.0429(2), c = 13.2004(2) Angstrom, and 12 water/unit cell. During the phase transition at about 540 K the mixed CaA/Na site is separate into distinct CaA and Na sites. As the Na sites do not require water coordination another 1/6 of the original water contents may be expelled. Thomsonite, heat treated at 583 K: Spacegroup P11n, a = 12.9890(2), b = 13.2016(2), c = 12.7646(2) Angstrom, gamma = 89.434(1)degrees and 8 water/unit cell. During this phase transition the CaB move closer to each other to enable sharing of two waters as for CaA. Another 1/6 of the original water contents may then be expelled.