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The diffusion couple was used to study solid state reaction in the FexO-TiO2 system at a temperature range of 1323 to 1473 K,during which the oxygen partial pressure was controlled by CO-CO2 gas mixture. The cross-sections of the diffusion couple were observed by electron probe microanalysis(EPMA),and the diffusion concentration profiles of Fe2+,Ti4+,and O2- ions were also determined. In combination with the phase diagram of the Fe O-TiO2 system,it was found that ulvospinel,ilmenite,pseudobrookite phases formed during the reaction and their growth was diffusion-controlled. According to the concentration changes of diffusion ions,the diffusion path of Fe ions was plotted,based on which the solid state reaction mechanism of this system was reasonably proposed.