The compression of water-insoluble drug monolayers spread on the aqueous subphase containing cyclodextrins (CD) led to a shift of surface pressure (pi)-area (A) isotherms toward smaller molecular areas with respect to the pi-A isotherms on the pure water subphase. The displacement of the compression isotherm obtained for the retinol spread on the beta-CD containing aqueous, subphase was used to quantify the depletion process and to determine the drug-CD association constants. The proposed method appeared to be sensitive enough to account for extremely low amounts of sequestered drug molecules. The obtained values of the association constants K-a ranged from about 1.4.10(-2) to 36 m(2)/mol. The magnitudes of these constants are discussed in terms of drug bioavailability and of the stoichiometry of retinol-beta-cyclodextrin inclusion complex which was shown to have a 1:1 correspondence.