Two-dimensional fluoride ion conductor KSn2F5 undergoes a first-order phase transition to a superionic state at T-tr = 428 K associated with DeltaS(tr) = 6.3 J K-1 mol(-1). The structural changes in the temperature range from 200 to 450 K, including the superionic phase transition, were investigated by means of single-crystal X-ray analysis and Rietveld analysis of the powder data. The intrinsic fluoride ion vacancies formed in the anionic layer contribute to the high ionic conductivity at Phase II (10(-5) S cm(-1) at 300 K). On the other hand, a two-dimensional average structure appears at Phase I due to the formation of the dynamically disordered F- sites in the anionic layer. The stepwise expansion in the ab plane at T-tr also supports this highly disordered structure. The charge density distribution derived from the MEM analysis at 450 K clearly showed a two-dimensional conduction path. (C) 2003 Elsevier B.V. All rights reserved.