The syntheses of nanosize magnetite particles by wet-chemical oxidation of Fe(2+) have been extensively investigated. In the present investigation the nanosize magnetite particles were synthesised without using the Fe(II) precursor. This was achieved by gamma-irradiation of water-in-oil microemulsion containing only the Fe(III) precursor. The corresponding phase transformations were monitored. Microemulsions (pH similar to 12.5) were gamma-irradiated at a relatively high dose rate of similar to 22 kGy/h. Upon 1 h of gamma-irradiation the XRD pattern of the precipitate showed goethite and unidentified low-intensity peaks. Upon 6 h of gamma-irradiation, reductive conditions were achieved and substoichiometric magnetite (similar to Fe(2.71)O(4)) particles with insignificant amount of goethite particles found in the precipitate. Hydrated electrons (e(aq)), organic radicals and hydrogen gas as radiolytic products were responsible for the reductive dissolution of iron oxide in the microemulsion and the reduction Fe(3+) -> Fe(2+). Upon 18 h of gamma-irradiation the precipitate exhibited dual behaviour, it was a more oxidised product than the precipitate obtained after 6 h of gamma-irradiation, but it contained magnetite particles in a more reduced form (similar to Fe(2.93)O(4)). It was presumed that the reduction and oxidation processes existed as concurrent competitive processes in the rnicroemulsion. After 18 h of gamma-irradiation the pH of the medium shifted from the alkaline to the acidic range. The high dose rate of similar to 22 kGy/h was directly responsible for this shift to the acidic range. At a slightly acidic pH a further reduction of Fe(3+) -> Fe(2+) resulted in the formation of more stoichiometric magnetite particles, whereas the oxidation conditions in the acidic medium permitted the oxidation Fe(3+) -> Fe(2+). The Fe(3+) Was much less soluble in the acidic medium and it hydrolysed and recrystallised as goethite. The gamma-irradiation of the microemulsion for 25 h at a lower dose rate of 16 kGy/h produced pure substoichiometric nanosize magnetite particles of about 25 nm in size and with the stoichiometry of Fe(2.83)O(4). (C) 2009 Elsevier Ltd. All rights reserved.