In order to analyze the coupling effect in thermodynamic swelling process, the relationship between the activity and volume fraction of each component within membrane was defined as alpha(i)(m) = gamma(i)(m) phi(i)(m). The concept of reference state in thermodynamic swelling process which only considered the interaction between individual component and membrane matrix was also set up. The difference of activity coefficient of each component within the membrane from reference state reflected the contribution of coupling effect to thermodynamic swelling process. For chitosan and H2SO4 crosslinking chitosan membrane the influence of coupling effect on selectivity of H2O in thermodynamic swelling process could be described from distribution coefficients and competition distribution coefficients of H2O and EtOH successfully. The interaction between H2O and the membrane matrix dominated the activity coefficient of H2O within the membrane, and the coupling effect between H2O and EtOH within the membrane only had a little influence on the activity coefficient of H2O. However, the activity coefficient of EtOH was dominated strongly by the coupling effect between H2O and EtOH within the membrane. This was attributed to the large amount of H2O present in the membrane. The distribution coefficient of EtOH, k(2)(m) and competition distribution coefficient of membrane k(12)(m) directly determined the selectivity of H2O in thermodynamic swelling process and the coupling effect between H2O and EtOH within the membrane had a negative influence on the selectivity of H2O within the membrane.