Steady-state fluorescence and molecular mechanics calculation were used to study the inclusion complexes of 2-methylnaphthoate (MN) with gamma-cyclodextrin (gamma-CD). The stoichiometry (I:I) and binding constants (213 +/- 96 M-1 at 25 degrees C) were extracted from an analysis of the ratio of the intensity of two emission bands that were sensitive to the polarity of the medium. Extrapolation of this ratio to a high concentration of gamma-CD permits the estimation of the polarity of the inner cavity, which seems to be only slightly hydrophobic, with a dielectric constant near 74. The Delta H and Delta S for formation of the complex were obtained and compared with previous results for similar complexes of MN with two smaller CDs. Molecular mechanics calculations were applied to study the complex in vacuo and in the presence of water as a solvent. Complexation is mainly due to nonbonded van der Waals interactions of MN with gamma-CD. Calculations show that MN penetrates completely into the cavity of gamma-CD.