The thermochemical changes taking place in the phase of a Fe(III)-containing strongly basic anion exchanger (AV-17) are investigated by using Mossbauer, IR, and EPR spectroscopy. During the heating of Fe(Ill)-containing resin in air up to 200 degrees C, redox processes in the polymer phase take place. On boiling in water, the jarosite-type compounds in the polymer are converted into higher dispersed beta-FeOOH particles. After sorption-boiling in water cycles in the polymer phase, relatively massive and magnetically ordered beta-FeOOH particles are formed. A portion of the beta-FeOOH particles remains in the highly dispersed superparamagnetic state distributed in the narrow pores of the exchanger. It is suggested that the Fe(III) ion sorption from Fe-2(SO4)(3) solutions by an exchanger takes place through the formation in the polymer phase of solid ultrafine particles of jarosite mineral-type compounds. (c) 2005 Wiley Periodicals, Inc.