The process of diffusion of one component (A) into another (B) in which their respective molar fluxes are equal but in opposite directions. The diffusivity of A in B is therefore the same as B in A in which the total pressure within the process remains the same throughout. The process of steady-state equimolar counter-diffusion in an ideal gas mixture can be described by Fick’s law for which the concentrations of A and B are expressed in terms of their partial pressures.