The relations between the mole fraction and the activity coefficient in the adsorbed phase were obtained from the thermodynamic analysis on the adsorption equilibrium of liquid mixtures. the asorption equilibroium data of the liquid mixtures, benzene-cyclohexane and tolune-cyclohexane, on the zeolite Na y and Na X were measured at 30 ℃. the activity coefficients in the adsorbed phase, calculated from the adsorption equilibrium data, were thermodynamically consistent. It was found that the amout of preferentially adsorbed molecules could be calculated from the experimental adsorption equilibrium data.