The use of Mössbauer spectroscopic data on the second-order Doppler (SOD) shift to determine the reduced isotopic partition function ratio (β-factor) has been considered by the example of iron. Using the relation between the β-factor and the SOD shift in Mössbauer spectra, the temperature dependence of the iron β-factors for a wide range of minerals has been evaluated from experimental data on the SOD shift. It is shown that the β-factors of Fe 3+ ions are considerably higher than those of Fe 2+. The curve describing the temperature dependence of the β-factor for native iron is the boundary separating fields that are typical for ferric and ferrous ions. The value of the iron β-factor increases with increasing covalence of chemical bonds. In the case of covalent chemical bonds, the iron β-factor achieves high values even for ferrous compounds. Possible iron isotope geothermometers magnetite–siderite and pyrite–siderite have been calibrated 10 3 ln β magnetite–siderite=0.881 776 x−0.544 105×10 −2 x 2+0.425 639 10 −4 x 3−0.352 191×10 −5 x 4, 10 3 ln β pyrite–siderite=0.913 717 x−0.557 721×10 −2 x 2+0.424 146×10 −4 x 3−0.334 281×10 −5 x 4, where x = 10 6/ T 2, T is absolute temperature, ln β relates to 57Fe/ 54Fe fractionation. At equilibrium, a small iron isotopic shift between magnetite and pyrite along with high iron isotopic shifts between magnetite and siderite and between pyrite and siderite should be observed. A significant effect (about 7‰ at 300 K) of the aluminum substitution on the iron β-factor in hematite has been evaluated from the appropriate data on the SOD shift in Mössbauer spectra. The analogous effect of the Co-substitution in magnetite is lower (≈3.0‰ at 300 K). A new method of evaluation of the β-factor for isotopes traditionally used in geochemical studies like sulfur, oxygen, etc., is suggested. The method uses experimental Mössbauer data on the SOD shift and calorimetric data on the heat capacity. The method can be applied to compounds consisting of two chemical elements (like oxides, sulfides) if one of them has a Mössbauer-sensitive isotope. Using the new method, the β 34S-factor of pyrite and the β 18O-factor of hematite have been determined: 10 3 ln β pyrite=(1.5997±0.0419) x−(6.7744±0.4279)×10 −3 x 2+(3.8254±0.5682)×10 −5 x 3, 10 3 ln β hematite=(5.7215±0.3891) x−(0.029 41±0.004 49) x 2.