Abstract The inclusion complexes of cyclomaltoheptaose (β-CD) and β-CD's 50% hydroxyethyl-substituted derivative (HE-β-CD) with chlorogenic acid (CA) were studied with regard to temperature and water activity ( a H 2O ≈ mole fraction = X H 2O = 0.8–0.99; 0.1 M Na phosphate buffer) utilizing first-derivative spectrophotometric analyses of bathochromic shifts (Δλ) in CA's UV absorbance as a function of variable [CD]. From the dependence of the apparent stability constant, K, on X H 2O (K = K ‡X H 2O z) we estimated that the β-CD · CA complex's apparent stoichiometric coefficient, z, for water was ca. 7 ± 1 (K ‡ = 1032 ± 54 M −1) ; this value agrees with recently published literature concerning the minimum number of waters needed to stabilize a similar β-CD adduct. However, we determined that z was significantly lower ( 4 ± 0.3; K ‡ = 809 ± 31 M − ) for the HE-β-CD · CA complex. These results argue that a unique species of bound water is involved in β-CD · CA stability since a 50% substitution resulted in an equivalent loss in z as well as substantial decrease in K ‡ . This hypothesis was supported by NMR inversion recovery experiments whereupon the most significant perturbation to spin-lattice relaxation ( ΔT 1 = T 1 β- CD − T 1 β- CD · CA ) was associated with β-CD's 1H at position 3 (H−3; ΔT 1 = 585 ms). Small ΔT 1s were also observed for H-2 (160 ms) and H-6,6′ (83 ms). β-CD's ΔT 1s were dependent not only upon the adduct's concentration but also diminished at a high ionic strength. These data indicate that ΔT 1 was related to changes in [D 2O] at or near β-CD's hydroxyl groups and that these D 2O molecules were bound with a relatively long residence time. Thermochemical measurements of ΔH and ΔS at various X H 2O s display typically linear enthalpy-entropy compensation ( ΔH− ΔS) relationships but with a slope ( T c = ∂ΔH/ ∂ΔS = 272 K) significantly less than standard aqueous thermodynamic measurements ( T c = 305 K) of a similar system. This unequivocal X H 2O effect on T c argues that the chemical part process of CD · guest adduct formation involves changes in relative solvation, presumably desolvation, of β-CD's binding site. This interpretation was supported by the dependency of Δλ max on CD binding site dimension and X H 2O MeOH.