The interactions between bile acid anions and cyclodextrins (CDs) in aqueous solution at room temperature have been studied in detail by the NMR titration method. The conformation adopted by the inclusion complex depends on the size of the cavity of the CD : The inclusion complexes with beta-CD result primarily from interactions with the smaller, hydrophilic side chains of the bile acid anions, while those with gamma-CD, which has a larger cavity, involve interactions with the larger, hydrophobic end of bile acid anions. The results indicate that hydrophobic interactions between the host CD molecules and the guest bile acid anions are important in the formation of the complexes. Steric hindrance of certain groups, especially that due to the 12-OH group of the steroid skeleton, has important effects on the interaction. These results are consistent with additional information derived from molecular modeling studies.