A large Seebeck coefficient (Se) of 1.9 mV K-1 was recorded for the I-/I3- thermocell by utilizing the host-guest complexation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin (Me18-α-CD) with the oxidized iodide species. The thermocell measurement and UV-vis spectroscopy unveiled the formation of an Me18-α-CD-pentaiodide (I5-) complex, which is in remarkable contrast to the triiodide complex α-CD-I3- previously reported. Although the precipitation of the α-CD-I3- complex in the presence of an electrolyte such as potassium chloride is a problem in thermocells, this issue was solved by using Me18-α-CD as a host compound. The absence of precipitation in the Me18-α-CD and I-/I3- system containing potassium chloride not only improved the Se of the I-/I3- thermocell, but also significantly enhanced the temporal stability of its power output. This is the first observation that I5- species is formed in aqueous solution in a thermocell. Furthermore, the solution equilibrium of the redox couples was controlled by tuning the chemical structure of the host compounds. Thus, the integration of host-guest chemistry with redox couples extends the application of thermocells.