The effect of bulk flow normal to the interface on the interphase mass transfer rate is analyzed by using a convective mass transfer coefficient, and a method of deriving the correct choice of the reference mole fraction in the bulk flow term is developed. The effect of the bulk flow and the definition of the proper reference mole fraction have been derived based on diffusion in a binary gas mixture across the boundary layer and expanded to the system of an interfacial gas-solid reaction. The results show that the bulk flow effect together with the proper choice of the reference mole fraction in the bulk flow term is important in obtaining an accurate expression of interphase mass transfer rate. In an extreme situation, the bulk flow effect can cause mass transfer to occur in the direction of increasing concentration. The theoretical development is applied to the rate analysis of the hydrogen reduction of silica.