Although the complexation of carborane derivatives with beta-cyclodextrin (beta-CD) is well-known, we present here the first observation of the complexation of carborane derivatives with alpha-CD in aqueous solution. The stoichiometry and association constant (K(a)) of the complexes were estimated from Job's plots and NMR titration studies, respectively. The carborane : alpha-CD stoichiometry was 1 : 1 in all cases. The complexation ability and selectivity of the carborane derivatives for alpha-CD are markedly decreased compared with those for beta-CD. The interaction between the carborane cage and the hydrophobic cavity of alpha-CD appears to be weak, probably because the hydrophobic cavity of alpha-CD is too small to accommodate the carborane cage. The C-H hydrogen and the polar substituents of carborane cage may form hydrogen bonds with secondary alcohol groups at the rim of alpha-CD. The orientation of the carborane cage influences the inclusion process, and o- and m-carborane derivatives showed moderately stronger association constants than p-carborane derivatives.