The scarcely reactive zinc ferrite can be decomposed in solid phase to water-soluble ZnSO4 and water-insoluble Fe2O3 polymorphs by excess of solid iron(II or III) sulphates at 650°C in 4 h. Since SO3 forms in situ from the Fe2(SO4)3 as well as anhydrous Fe2(SO4)3 forms in the thermal oxidation of iron(II) sulphate, therefore the gaseous SO3 is the active sulphatization agent in the reaction of the hydrated iron(II and III) sulphates and the zinc ferrite. The sulphatization process were monitored by XRD, TG/DTA-MS, Mössbauer, Raman and IR-spectroscopy and the changes in reaction’s heat and Gibbs energy are reported. The Fe2O3 polymorphs formed is worth processing into iron(III) sulphate by sulphuric acid. Since the Fe2(SO4)3 formed in this way can be recycled to the sulphatization process, the only reagent consumed is the sulphuric acid. Therefore, these selective sulphatization reactions can be taken as “iron salts mediated” decompositions of zinc ferrite into ZnSO4 and Fe2O3 by sulphuric acid.