The dependence of the equilibrium concentration of a dilute aqueous solution of I-propanol on the ratio of the activated carbon mass to the mass of the solution phase was investigated at fixed initial concentrations of the solution. It was proved that the adsorbed amount depends not only on the "equilibrium" concentration but on the initial concentration of the solution and on the sorbent "concentration" (ratio of adsorbent mass to the mass of the solution) in the investigated systems. On the other hand, the equilibrium;adsorption isotherms of binary liquid mixtures must be an unequivocal function of the equilibrium concentration at constant temperature and pressure. This contradiction was clarified by studying the conditions referring to the rigorous definition of equilibrium physical adsorption of binary liquid mixtures onto solids. It was shown that the investigated adsorption systems are not in equilibrium but in different metastable states and the sorbent "concentration" effect could be interpreted as a consequence of this. Finally it can be concluded that one must take special care in interpreting the surface excess isotherms of aqueous mixtures on porous adsorbents as equilibrium quantities since the time-independent supernatant concentration (at constant pressure and temperature) does not mean equilibrium state without any doubt.