Complex stability constants (Ks), standard molar enthalpic changes (Delta H degrees), and entropic changes (T Delta S degrees) for the inclusion complexations of native beta-cyclodextrin (1) and two oppositely charged P-cyclodextrins, i.e., mono(6-amino-6-deoxy)-beta-cyclodextrin (2) and mono [6-O-6-(4-carboxylphenyl)] -beta-cyclodextrin (3), with two (ferrocenylmethyl)dimethylaminium derivatives, i.e., FC4(+)Br(-) and FC8+Br-, were determined at 25 degrees C in aqueous phosphate buffer solution (pH 7.20) by means of isothermal titration microcalorimetry (ITC). Cyclic voltammetry studies showed that the ferrocene groups of the guests were included in the beta-cyclodextrin cavity to form host-guest complexes. As compared with neutral beta-cyclodextrin, the positively charged host 2 showed decreased binding toward (ferrocenylmethyl)dimethylaminium guests. This was attributed to electrostatic repulsion, while the negatively charged host 3 displayed increased binding due to electrostatic attractions. Thermodynamically, the ionization of host CDs affects both enthalpic and entropic changes of host-guest complexations presumably by changing the hydrophobicity and the desolvation effect of hosts upon inclusion complexation. Moreover, the solvent effect was also discussed from the viewpoint of thermodynamics.