This work studies the interaction of substituted coumarins with fi-cyclodextrin W-CD). The role of the substituent in the 3 position was investigated. The guest molecules were two original 4-hydroxycoumarin derivatives, bearing an ethyl furoate (1) or a benzodioxanyl (2) substituent in the 3 position. Comparison was made with a nonsubstituted analogue (3). It must be noted that molecules 1 and 2 potentially offer very different complexation sites. Formation of the inclusion complex was checked by UV/vis absorption and fluorescence spectroscopy. A strong revival of the fluorescence intensity of 1 and 2 was observed in the presence of beta -CD and partly attributed to the fact that the heterocyclic substituent was prevented from rotating freely. The 1:1 stoichiometry of the complex was established, and the values of the binding constants (7 x 10(2) and 3.4 x 10(2) M-1 for I and 2, respectively) were extracted from the spectrophotometric data. A much lower (8.1 x 10(1) M-1) binding constant was found for unsubstituted compound 3. The H-1 NMR study confirmed that the coumaryl ring system was engulfed in the cyclodextrin cavity. The proximal moiety of the heterocyclic substituent also interacted with the beta -CD, whereas the rest protruded outside the cavity. This explains that the substituent controls the structure of the inclusion complex and contributes to its stabilization.