| Summary: | Here we focus on the thermal and variable temperature electrochemical stabilities of two ionic liquids (ILs) having a common tributyloctyl phosphonium cation [P<sub>4,4,4,8</sub>]<sup>+</sup> and two different orthoborate anions: bis(mandelato)borate [BMB]<sup>−</sup> and bis(salicylato)borate [BScB]<sup>−</sup>. The thermo-gravimetric analysis data suggest that [P<sub>4,4,4,8</sub>][BScB] is thermally more stable than [P<sub>4,4,4,8</sub>][BMB] in both nitrogen atmosphere and air, while the impedance spectroscopy reveals that [P<sub>4,4,4,8</sub>][BScB] has higher ionic conductivity than [P<sub>4,4,4,8</sub>][BMB] over the whole studied temperature range. In contrast, the electrochemical studies confirm that [P<sub>4,4,4,8</sub>][BMB] is more stable and exhibits a wider electrochemical stability window (ESW) on a glassy carbon electrode surface as compared to [P<sub>4,4,4,8</sub>][BScB]. A continuous decrease in the ESWs of both ILs is observed as a function of operation temperature.
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