Opposite migration order was observed for the enantiomers of the chiral beta (2)-adrenergic drug clenbuterol (CL) in capillary electrophoresis (CE) when resolved with native beta -cyclodextrin (beta -CD) and heptakis (2,3-diacetyl-6-sulfo)-beta -CD (HDAS-beta -CD). The possible mechanisms of the affinity reversal of the CL enantiomers depending on the structure of the CD were studied using H-1-nuclear magnetic resonance (H-1-NMR) spectrometry and one-dimensional rotating frame nuclear Overhauser and exchange spectrometry (1-D ROESY). Significant differences were observed between the structure of the (+/-)-CL complexes with beta -CD and HDAS-beta -CD.