The self‐diffusion isotope effect has been measured by serial sectioning in the &agr;, &ggr;, and &dgr; phases of iron. Values for the isotope effect assuming a single vacancy exchange are 0.71±0.06 at 1452°C (bcc); 0.79±0.05 at 1338°C (fcc); 0.69±0.14 at 869°C (bcc). Thus, in each case the isotope effect equals the correlation factor within experimental error. Measurements at 720° and 770°C appear to be influenced by diffusion along grain boundaries producing a marked reduction in the apparent isotope effect.