Ethylated α-cyclodextrin (Et18-α-CD) is used as a host matrix for I-/I3- thermocells. Although Et18-α-CD is not soluble in water at ambient temperature, it becomes soluble by complexation of the I3- anion. Meanwhile, the complex is precipitated upon elevating the temperature. The change in thermo-responsive solubility of the I3-/Et18-α-CD complex increases the Seebeck coefficient (Se) of the thermocell up to 2.6 mV K-1. The underlying mechanism of the increased Se is elucidated by UV-vis spectroscopy, Raman spectroscopy, and electrochemical measurements. This result shows the temperature-dependent solubility changes of redox-active species as a potential means to improve the performance of electrochemical thermocells. This journal is