This work reports on the chemical isotope fractionation of Fe(III) by a solvent extraction method with a crown ether of dicyclohexano-18-crown-6. The Fe-56/Fe-54 and Fe-57/Fe-54 ratios were analyzed by multiple-collector inductively coupled plasma mass spectrometry. We determined the dependence of the isotope enrichment factors (epsilon) on the strength of HCl. The relative deviation of the Fe-56/Fe-54 ratios relative to the unprocessed material (10(4) epsilon(56)) increases from -15.3 to -6.3 with [HCl] increasing from 1.6 to 3.5 mol/L. Likewise, 10(4) epsilon(57) increases from -22.8 to -9.6 under the same conditions. The correlation between epsilon(56) and epsilon(57) is mass dependent within the errors. The observed fractionation was broken down into the effects of competing extraction reactions and of a reaction between Fe(III) species (FeCl2+ and FeCl3) in the aqueous phase. We found that the isotope fractionation between the Fe(III) species is mass dependent, which we confirmed by calculating the reduced partition function ratios.