The interaction of two biliar acids (chenodeoxycholic acid and cholic acid) with 2-hydroxypropyl-beta-cyclodextrin (HP beta CD) in solution and in the solid state was studied using different techniques. The formation of an inclusion complex with a 1 : 1 stoichiometry was suggested by the phase solubility studies. Both differential scanning calorimetry and X-ray diffractometry exhibited the amorphous state of the complex. The inclusion of both biliar acids into the HP beta CD cavity was confirmed by the C-13-NMR studies. Cholic acid showed a weaker affinity with respect to chenodeoxycholic acid probably owing to the presence of a hydroxyl group on C(12) (12 alpha) close to the complexation site.