To understand the nature of the rate-controlling process in reduction of iron oxide with hydrogen, the effective diffusivity for the equimolar counterdiffusion of hydrogen and water vapor through the porous iron hollow cylinder were measured at temperature from 700 ℃ to 1,000 ℃. The ratio of effective diffusivity to molecular diffusivity, D_eff/D_H2-H2O, increase with increasing reduction temperature. At 1,000 ℃, the diffusivity ratio approaches 0.35, the value calculated for an idealized pore structure. The activation energy of diffusion is 16.49 kcal. The reduction rate of iron oxide with hydrogen was shown to be limited to a great extent by the diffusion of hydrogen and water vapor through the porous iron layer.