Abstract: The enrichment of chromium in the magnetic iron chromite (Fe(Cr_xFe_1−x)₂O₄) phase is crucial for the recovery and recycling of chromium in stainless-steel pickling sludge. The kinetics and reaction mechanism of the solid-phase reaction between Fe₃O₄ and Cr₂O₃ were investigated using the diffusion couple method at 1473 K. Not only the diffusion behavior of Fe²⁺ ions and Cr³⁺ ions was elucidated, but also the solid solution behavior of Fe³⁺ ions was discussed clearly. The microscopic morphology of the diffusion couple and the change in the concentrations of Fe and Cr cations across the diffusion layers were analyzed using scanning electron microscopy and energy dispersive spectroscopy. The self-diffusion coefficients of cations were calculated based on the concentration profiles of Fe and Cr, with the results indicating that the self-diffusion coefficient of the Fe ions was consistently higher than that of the Cr ions. Additionally, a mixture of Fe₃O₄ and Cr₂O₃ was annealed at 1373–1473 K for 1–5 h, and the kinetic parameters were calculated by studying the phase content of the product. The phase content of Fe(Cr_xFe_1−x)₂O₄ in the product was determined by Rietveld refinement of X-ray diffraction data, revealing that an activation energy (E) of 177.20 kJ·mol⁻¹ and a pre-exponential factor (B) of 610.78 min⁻¹ of the solid-phase reaction that produced the Fe(Cr_xFe_1−x)₂O₄ spinel.