Behavior of Fe_1-xNi_x alloy catalysts during the hydrogenation of CO was studied by Mossbauer spectroscopy and X-ray diffraction analysis. The behavior depends on the crystal structure and composition of the alloy. In a case x=0, α Fe was converted into ε' Fe_2.2C and χ Fe₅C₂, and after a long period of catalyst utilization χ Fe₅C₂ became a majority of the catalyst. Bcc Fe_1-xNi_x (0<x≤0.20) was converted into ε' (Fe_1-xNi_x)_2.2C, whereas no carbides were formed from fcc Fe_1-xNi_x (0.55≤x≤0.85). Carbon atoms solved into the fcc lattice and formed a fcc solid solution, Fe_1-xNi_xC_y. Ni was stable during the catalyst utilization.