Four allotropic forms are shown for the K3InF6 ternary fluoride by various experimental investigations, microcalorimetry, high temperature X-ray diffraction, optical and electrical measurements. The ac conductivity data of K3InF6 are analysed in the Z*, M* and epsilon* formalisms and the conductivity relaxation parameters are determined. Conductivity is of Arrhenius-type on either side of a transition temperature T-tr (T-tr = 488 +/- 10 K) which is very close to the Curie temperature T-C (T-C = 480 +/- 5 K). A conductivity jump of about one decade is observed at T-C and that transition is of ionic-superionic type. (C) 2000 Elsevier Science B.V. All rights reserved.