Abstract The incorporation of radioactive elements derived from fission products (FPs) into a stabilized crystalline structure is an active area of research in the nuclear fuel cycle. The compound Sr2Fe1.5Mo0.5O6−δ (SFM) incorporates several FPs (Sr and Mo) into the crystalline network simultaneously while maintaining a stabilized perovskite structure. This polycrystalline SFM sample was irradiated with 200keV He ions to a fluence of 5×1016ionscm−2 (corresponding to a peak dose at 1.2dpa) at room temperature to study radiation damage effects. Irradiated SFM sample decomposed into a layered Sr4FeMoO8−δ based phase and a Fe metallic phase, resulting in nano-crystalized grains with particle sizes around 7nm. It was rationalized that He ion irradiation created a reduced atmosphere on the surface of SFM sample by preferentially sputtering oxygen atoms, consequently decomposing SFM and reducing Fe ions into metallic Fe phases. These results suggest that the stability of crystalline structures in reducing atmospheres may be an additional factor for consideration in the search for FP hosts in crystalline materials.