Coupled transport phenomena across a gas/liquid interface, relevant for distillation, were studied by nonequilibrium molecular dynamics simulations. The simulations were set in the context of bulk irreversible thermodynamics. It was then shown that mole fraction profiles in the liquid phase and the gas phase of ideal isotope mixtures are linear. For nonideal mixtures, Fick’s law cannot be applied in the interface region, because the activity coefficients change dramatically across the interface. Fourier’s law has a constant heat conductivity for both types of liquid mixtures but not for gas mixtures. The coupling between heat and mass transfer becomes negligible for distillation in the special case of ideal mixtures with constant molal overflow. In all other cases, the heat of transfer contributes significantly to the heat flux and causes deviations from Fourier’s law in the gas phase. This all means that coupled flux equations are needed to describe distillation and that the properties of the surface are important for a description of the heat and mass fluxes involved. The value of the heat of transfer has a bearing on the calculation of the number of theoretical stages in the column. When considered as a function of distance from the surface, the local entropy production rate has a peak or a shoulder (depending on the conditions) slightly into the vapor. The entropy production rate in the liquid cannot be neglected compared to that of the gas. The second law efficiency of distillation was quantified from this knowledge.