In a fluidized catalyst bed, defluidization occurs when the fluidizing gas is switched from lower to higher density gases. This phenomenon occurs when the gas velocity in the emulsion phase falls below the minimum fluidizing velocity due to non-equimolar counter diffusion between the emulsion phase and the bubble phase. Non-equimolar diffusion in small pores under isobaric condition is known as Graham s law, and many models have been proposed since the mid-20-century. In these models, non-equimolar counter diffusion is explained using the binary diffusion coefficient, because the phenomenon occurs in the molecular diffusion region. In this study, equimolar counter diffusion is not assumed when the pore size is small even in the molecular diffusion region; rather, according to the recognized mechanism of equimolar diffusion, it is explained by the intrinsic diffusion of each molecule and the flow caused by the pressure gradient. To confirm this explanation, a model was proposed that includes an equation for the intrinsic diffusion coefficient. Calculations based on this model showed good agreement with previously reported experimental data on the binary gas diffusion in pores under isobaric or isovolumetric conditions.