A highly unusual structural evolution has been observed in temperature dependent studies of the fast ion conductor Ag7P3S11, using X-ray diffraction, Raman scattering, P-31 and Ag-109 NMR spectroscopy, and electrical conductivity measurements. At 205 K the high-temperature gamma-phase (space group C2/c) undergoes a phase transition to an intermediate beta-phase of different symmetry. At a temperature near 130 K another phase transition is observed resulting in the formation of an ordered low-temperature alpha-modification crystallizing in the same space group as the gamma-phase. Restoration of the high-temperature-phase symmetry in the low-temperature phase is unambiguously confirmed by single-crystal X-ray structure determination and 31 p solid state NMR peak multiplicities. The re-entrant phase behavior is further supported by temperature dependent electrical conductivity measurements, which reveal that the activation energies of the dc conductivity for the alpha- and gamma-phases are identical and significantly lower compared to those measured in the beta-phase. Although the beta- to gamma-phase transition is associated with a change in enthalpy, those observables reflecting silver ion dynamics show no discontinuities at the phase transition temperature. The high-temperature gamma-phase crystallizes in the monoclinic system, space group C2/c (No. 15), a = 23.999(2) Angstrom, b = 6.3621(3) Angstrom, c 24.909(2) Angstrom, beta=110.926(7)degrees R = 0.0318 (300 K). The low-temperature ce-phase is isostructural with a = 24.090(1) Angstrom, b = 6.3400(3) Angstrom, c = 24.581 (1) Angstrom, beta = 110.870(6)degrees, R = 0.0317 (120 K). Contrary to the situation in gamma-Ag7P3S11, all the silver atoms are well-localized in the a-phase. (C) 2004 Elsevier SAS. All rights reserved.