A thermocell with use of the electrochemical Seebeck coefficient (alpha dEdT;E and T are the redox potential and temperature, respectively). is a promising energy-harvesting device. alpha is the key parameter that governs the thermal efficiency (eta) of the thermocell. Here, we systematically investigated the variation (Delta alpha) in alpha of a redox couple of [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) in water, by adding 9% organic solvent in molar ratio. We found that Delta alpha for 11 organic solvents well scale to the molar volume (V). The empirical volume effect is understood in terms of the replacement of water molecules with organic molecules depending on V. (c) 2020 The Japan Society of Applied Physics