The effects of Kr ion irradiation of FexCo1-x/Cu multilayers are investigated by means of magnetic and x-ray measurements. The irradiation was performed at room temperature with 600 keV of Kr, and the fluences were ranged from 1×1015 to 5×1015 ions/cm2. X-ray diffraction patterns show that the irradiation was able to produce a significant grain growth and, at the same time, it triggered a strain release. The x-ray absorption measurements around Fe K edge have shown that the FeCo environment changed from bcc, for the as-deposited multilayer, to fcc after appropriate fluence, depending on the Cu thickness and on the Fe/Co content. For 50 Å of Cu, the phase transformation occurs even for the lowest fluence, regardless the Fe/Co concentration. For 25 Å of Cu, it depends on the Fe/Co concentration. After irradiation, the saturation magnetization suffers a strong decrease, especially for 50 Å of Cu. The temperature for the onset of irreversibility also decreases with irradiation, again being the effect stronger for 50 Å of Cu. These data indicate that Kr irradiation induced mixing between Cu and FeCo, even though Cu is immiscible with Fe and Co in the equilibrium state.