High-resolution powder diffraction was used to study the thermal transformation of silver carbonate. A sample of Ag2CO3 was heated in a capillary under 4.5 atm CO2 pressure. The decomposition temperature of silver carbonate to silver oxide is thereby increased, allowing high-resolution synchrotron X-ray powder diffraction patterns of the two high-temperature phases of Ag2CO3 to be collected. The structure of the low-temperature (lt) phase was confirmed, and the structures of the two high-temperature phases were determined by direct methods and refined using the Rietveld method: It-Ag2CO3 (295 K) P2(1)/m, z = 2, a = 4.8521(2) Angstrom, b = 9.5489(4) Angstrom, c = 3.2536(1) Angstrom, = 91.9713(3)degrees; beta-Ag2CO3 (453 K) P31c, z = 6, a = 9.1716(4) Angstrom, c = 6.5176(3) Angstrom; alpha-Ag2CO3 (476 K) P (6) over bar 2m, z = 3, a = 9.0924(4) Angstrom, c = 3.3249(1) Angstrom. In addition, thermal expansion properties, anisotropic microstrain distributions, and thermal transformations of the three silver carbonate phases and silver oxide are described.