In this work, the sorption and steady state permeation of water-ethanol mixture in an aromatic polyimide membrane was investigated with the objective of revealing the coupling effects of water and ethanol in their course of diffusion. The solubility of the water-ethanol mixture in the membrane was determined by means of swelling-distillation techniques. The sorption behavior was interpreted in terms of the thermodynamic equilibrium equations. The Maxwell-Stefan mass transfer model was employed as a framework for modeling diffusion. Model equations derived therefrom enable us to calculate coupled fluxes of water and ethanol from the experimental data. The coupling effects in the diffusion of water and ethanol were revealed by comparing the calculated coupled flux with non-coupling flux, which was calculated by integration of the Fickian first law using independent diffusion coefficients. Furthermore, the implication of the sole adjustable parameter included in the derived model equations was discussed.