A 1:1 complex of 6-O-alpha-D-glucosyl-beta-cyclodextrin (G(1)-beta-CD) with 4-(dimethylamino)benzonitrile (DMABN) associates with itself in aqueous solutions at high concentrations into a 2:2 complex (homodimer). The 1:1 complex of G(1)-beta-CD with DMABN also makes a heterodimer (1:1:2 complex) by the association with a 1:1 inclusion complex of another guest molecule such as 1-pentanol, benzene, anisole, and benzonitrile. The association of the complex is accompanied by a drastic change in the fluorescence spectrum of DMABN. The measurement of the fluorescence intensity as a function of the concentration of G(1)-beta-CD gave the equilibrium constants for the association. The equilibrium constants were greatly dependent on the temperature, and the values for both Delta H and Delta S were largely negative. The fluorescence spectra of DMABN in the cavity of the dimers changed with exchanging the second guest compound, which implies that the second guest molecule plays the role of a polar solvent in the nonpolar environment in the cavity. The formation of homo- and heterodimers of the 1:1 inclusion complex also takes place in the solutions containing other cyclodextrins and DMABN.