Electron-transfer reactions between iron and cobalt complexes were studied in beta-cyclodextrin (beta CD), 2-hydroxypropyl-beta-cyclodextrin (H beta CD), and 18-crown-6 ether (18C6) solutions. The results were rationalized taking as the basis the Marcus-Hush formalism. We employed two different approaches, depending on the kind of receptor and solvent, to obtain the reorganization and reaction free energies that determine the reaction rate constant. The opposite trends in reactivity observed in beta CD and H beta CD solutions and the behavior in solutions of 18C6 are explained.