The fluxes of a mixture of gases diffusing through a porous medium in the normal diffusion region are derived, with emphasis on the flux ratio at uniform pressure. The Boltzmann equation is solved by the moment method, putting boundary conditions in the solution. Maxwell's method is shown to be inadequate, and it is found that, besides the well‐known mass effect, a molecular interaction effect appears in the expression of the flux ratio. Thus, the experimental deviations from the usual law are explained. Then, the flux ratio is experimentally determined for an isobaricN2‐C2H4mixture and found to be in qualitative agreement with theory. Finally the experimental results on the motion of spheres in nonhomogeneous gaseous mixtures are explained for the first time.