Abstract The inclusion of dulcin in α- and β-cyclodextrin has been studied by fluorescence spectroscopy. To quantitatively describe complex formation between the β-cyclodextrin and dulcin, an association constant of 290 M −1 at 21° was obtained. The thermodynamics associated with the complex formation between dulcin and β-cyclodextrin in aqueous solution has been studied. The obtained value of Δ G 0 = −13.7 kJ/mole at 21°, together with Δ H 0 = −33.6 ± 2.3 kJ/mole and Δ S 0 = −67.2 ±8.3 Jmole −1 K −1 indicate that dulcin has a very marked tendency to associate with β-cyclodextrin in water. The inclusion complex of dulcin in β-cyclodextrin has been used to determine dulein in the range 0.13–5 μg/ml the method has been applied to determine dulcin in soft drinks.