Ion beam induced atomic mixing at Fe‐Mo interface has been studied by using the technique of conversion electron Mo¨ssbauer spectroscopy (CEMS). Use has been made of a thin (∼50 A˚) layer of Fe57isotope (enriched to 95.4%) at the interface to obtain the Mo¨ssbauer information selectively from this region. A noninterface sensitive measurement has also been performed to reveal the magnetic hyperfine interactions in the entire region of the iron overlayer. It is shown that a deposition induced reaction between the molybdenum substrate (having a thin coating of native oxide) and the Fe57layer renders a graded nature to the interface, which is transformed upon ion bombardment (100‐keV Kr+, dose ∼1016ions/cm2) into a disordered alloy. The dominant nonmagnetic component corresponding to the interface of the ion beam mixed sample happens to be a quadrupole doublet, which represents the presence of Fe57atoms in Fe2+charge state. The appearance of this contribution is attributed to formation of an oxygen coordinated ternary compound in the interface layer during ion bombardment. On the basis of the comparison of the results of the interface‐sensitive and non‐interface‐sensitive studies it is established that the inclusion of oxygen in the ion mixed sample is mainly confined to the near interface region and that the region of the overlayer contains a metastable alloy of the binary Fe‐Mo system. The influence of thermal annealing at various temperatures between 200 °C and 500 °C on the ion beam mixed state has also been studied by monitoring the changes in the hyperfine interaction parameters. It is shown that the oxygen‐incorporated regions of the disordered alloy retain the nonmagnetic Fe2+charge state subsequent to annealing at 500 °C, while other regions lead to precipitation of &agr;‐Fe and Fe2Mo phases.