The reaction zones formed in two kinds of diffusion couples: U–23at.%Zr/Fe and U–23at.%Zr/Fe–12at.%Cr, at 908, 923, 953, 973 and 988 K have been examined using the electron-probe microanalysis. In the U–Zr/Fe–Cr couples, diffusion of Cr to the U–Zr side is slower than that of Fe, and the Cr-rich phase is formed adjacent to the unreacted Fe–Cr alloy. Except for the Cr-rich phase, the measured compositions of the phases in the reaction zones in both U–Zr/Fe and U–Zr/Fe–Cr couples have corresponded well to those in the U–Zr–Fe ternary system. Each reaction zone can be divided to several layers. For the U–Zr side of the reaction zone, the configurations of the schematic diffusion paths, which are the curves connecting the average compositions of these layers on the U–Zr–( Fe+ Cr) composition triangle, are independent of the annealing temperature and the Cr addition to Fe. For the Fe(–Cr) side, however, the paths depend on the annealing temperature and the Cr addition to Fe. Some of the phases that are expected to emerge considering the schematic diffusion path and the U–Zr–Fe phase diagram have not been found at 988 K.