The crystal and molecular structures of isa-butyl(aqua)cobaloxime (I) and its hydrated inclusion complexes with alpha-cyclodextrin (II) and beta-cyclodextrin (III) were determined by X-ray diffraction analyses. Crystal data for I : P2(1)/c, a = 30.450(1) Angstrom; b = 9.659(2) Angstrom; c = 19.200(2) Angstrom; beta = 107.23(3)degrees; Z = 12. Crystals of II . H2O and III . 9H(2)O both crystallize in space group P2(1)2(1)2(1) With Z = 4; a = 13.847(1) Angstrom, b = 19.418(1) Angstrom, c = 24.593(1) Angstrom for II . 8H(2)O, and a = 28.188(6) Angstrom, b = 15.190(3) Angstrom, c = 19.017(4) Angstrom for III . 9H(2)O. In the inclusion complexes, the Co(DH)(2) unit is located near the wide opening of the cyclodextrin with its iso-butyl group inserted into the cyclodextrin cavity. The structure and conformation changes of the iso-butyl(aqua)cobaloxime are probably related to the noncovalent interaction between the iso-butyl group and cyclodextrin cavity, and steric interaction between cobaloxime and cyclodextrin rims. Our results also show that distortions of iso-butyl(aqua)cobaloxime/cyclodextrin induced by alpha-CD and beta-CD occur in different extent, which assumedly is due to the different cavity diameters of the cyclodextrins.