The prediction made in J. Appl. Phys.65, 3987 (1989) that the ferroelectric–ferroelastic phase transition atTc=490 K in K3Fe5F15will be accompanied by a change from order to disorder among the Fe2+/Fe3+ions has been confirmed by Mo¨ssbauer spectroscopy. The thermal dependence of both the Fe3+quadrupolar splitting and the Fe3+isomer shift exhibits a smooth decrease with increasing temperature toTc, each with a sharp and reproducible change in slope aboveTc. The average Fe2+quadrupolar splitting (QS) similarly undergoes a major decrease with increasing temperature, with a clear change in slope aboveTc. Furthermore, the thermal dependence of the Fe2+QS distribution shows that the Fe2+/Fe3+order required crystallographically belowTcis not conserved at higher temperatures. The proportion of Fe2+present belowTcis close to the stoichiometric 60% value; the proportion decreases aboveTcrapidly with temperature and becomes less than 50% by 543 K, consistent with increasing delocalization of an Fe2+electron.