Inclusion compounds of a macrocyclic cavitand cucurbit[8]uril (CB[8]) with cobalt(III) and nickel(II) complexes of 1,3-diaminopropane (tn) and 1,3-diamino-2-propanol (tmOH) (trans-[Co(tn)(2)Cl-2]@CB[8])Cl center dot 14H(2)O (1), [trans-[Co(tmOH)(tmO)]@CB[8])Cl-2 center dot 22H(2)O (2), and (trans-[Ni(tmOH)(2)]@CB[8]}Cl-2 center dot 22H(2)O (3) were synthesized and characterized by X-ray single crystal analysis, IR spectroscopy, ESI-MS, and by solid-state stripping voltammetry. The encapsulation of trans[Co(tn)(2)Cl-2](+) within the cavity of CB[8] stabilizes the complex toward ligand substitution reactions in aqueous solution. The electrochemical study demonstrates that CB[8] prefers the oxidized species in trans-[Co(tn)(2)Cl-2](+)/trans-[Co(tn)(2)Cl-2](0) and trans[Co(tmO)(tmOH)(2)](2+)/trans[Co(tmO)(tmOH)(2)](+) redox couples, but stabilizes the reduced form trans[Ni(tmOH)(2)](2+)against the oxidized species. The reversibility of voltammogram shapes evidence that for the inclusion compounds 1-3 electron transfer reactions proceed within the cavity of the host.