The temperature dependence of ionic conductivity was investigated under vibrational strong coupling, as shown in Figure . The third order of the cavity mode was detuned to the OH stretching mode by adjusting the cavity thick-ness to 3.3 µm. Temperature control was achieved using a custom-made cell. IR transmission spectra were obtained before and after the electrochemical measurements to con-firm the coupling state (detailed experimental conditions are described in the supporting information). The validity of the ionic conduction behavior was suggested even from the control experiments (Figure ).