The unprecedented selective encapsulation of trans-[Co(en)(2)Cl-2](+) from the mixture of trans and cis isomers into the cavity of macrocyclic cavitand cucurbit[8]uril (C48H48N32O16, CB[8]) leads to the inclusion compound {trans[ Co(en)(2)Cl-2]@CB[8]}Cl center dot 17H(2)O (1). Single-crystal X-ray analysis, H-1 NMR, and ESI-MS spectra confirm the formation of host-guest complex 1 in both solid state and solution. The geometry of the complex cation alters significantly upon inclusion, which causes appreciable hypsochromic shifts of the absorption bands of the guest complex. According to TGA data, inclusion of trans-[Co(en)(2)Cl-2](+) dramatically stabilizes the complex toward thermal decomposition. Encapsulation of trans- [Co(en)(2)Cl-2](+) into the CB[8] cavity also increases the stability of the metal complex toward isomerization into the cis form. The supramolecular adduct {cis-[Co(en)(2)(H2O)(2)](CB[8])(6)}Cl-3 center dot ca109.5H(2)O (2) was isolated only after prolonged heating of an aqueous solution of 1 at 120 C in an evacuated tube; it was characterized by X-ray crystallography, IR, and elemental analysis.