The interaction of 1-naphthalenesulfonate (1-NS) with β-cyclodextrin was investigated in a 0.1M phosphate buffer at pH7.4 by an LKB 2277 microcalorimeter, using a flow-mixed mode at 25°C. The thermodynamic parameters for inclusion complex formation obtained are as follows : ΔG°=-13.2kJ/mol (K=209M^-1), ΔH°=-1.87kJ/mol, ΔS°=38.1J/mol K. The main driving force for inclusion complex formation was considered to be the hydrophobic interaction, whereas the contribution of the van der Waals-London dispersion force was minor. Also, from measurements of the proton nuclear magnetic resonance spectra and the model building with Corey-Pauling-Koltum atomic models, the probable structures of the inclusion complex were determined.