Naturally occurring iron (₂₆Fe) consists of four stable isotopes: 5.845% of ⁵⁴Fe (possibly radioactive with a half-life over 4.4×10²⁰ years), 91.754% of ⁵⁶Fe, 2.119% of ⁵⁷Fe and 0.286% of ⁵⁸Fe. There are 24 known radioactive isotopes, the most stable of which are ⁶⁰Fe (half-life 2.6 million years) and ⁵⁵Fe (half-life 2.7 years).

Much of the past work on measuring the isotopic composition of Fe has centered on determining ⁶⁰Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.