A set of terrestrial geological samples with varying compositions of Fe2O3 + MeO.TiO2 (Me = Fe, Mg, Mn, etc.), distinguished by their Fe2O3 content (<25%), was studied using SIMS. The dependence of isotope fractionation upon the oxide phase content as well as the degree of iron oxidation was determined. It is shown that heavy isotopes fractionation is directly proportional to Fe2O3 content and the percentage of higher oxides in the sample. It is also shown that the fugacity of oxygen is a good general integral measure of these physical and chemical changes. It is concluded that the migration, redistribution, and accumulation of the iron group isotopes are controlled by the oxidation-reduction processes. Of particular note is the redistribution of oxygen between the local atomic environment of titanium and iron. The role of the ion-beam induced effects during isotopic SIMS analysis of these specimens is discussed. (C) 2004 Elsevier B.V. All rights reserved.