The electrochemical Seebeck coefficient (α = dV/dT; V and T are the redox potential and temperature, respectively) is important parameter for thermoelectric conversion. Here, we found that α of Fe²⁺/Fe³⁺ in dimethyl sulfoxide (DMSO) with small amount of 19 M LiCl aqueous solution exhibits a crossover behavior from 1.4 mV K⁻¹ (20 °C ≤ T ≤ 40 °C) to ≈0 mV K⁻¹ (50 °C ≤ T ≤ 80 °C). The molar absorption (ε) spectra revealed that the crossover is ascribed to the transformation of the dominant Fe³⁺ complex from [FeL₆]³⁺ (L is solvent molecule) to [FeCl₄]⁻.