Ionic liquids (ILs) composed of tetra(n-butyl)phosphonium [P-4444](+) and tetra(n-butyl)ammonium [N-4444](+) cations paired with 2-furoate [FuA]-, tetrahydo-2-furoate [HFuA](-), and thiophene-2-carboxylate [TpA]- anions are prepared to investigate the effects of electron delocalization in anion and the mutual interactions between cations and anions on their physical and electrochemical properties. The [P-4444](+) cations-based ILs are found to be liquids, while the [N-4444](+) cations-based ILs are semi-solids at room temperature. Thermogravimetric analysis revealed higher decomposition temperatures and differential scanning calorimetry analysis showed lower glass transition temperatures for phosphonium-based ILs than the ammonium-based counterparts. The ILs are arranged in the decreasing order of their ionic conductivities as [P-4444][HFuA] (0.069 mS cm(-1)) > [P-4444][FuA] (0.032 mS cm(-1)) > [P-4444][TpA] (0.028 mS cm(-1)) at 20 degrees C. The oxidative limit of the ILs followed the sequence of [FuA](-)> [TpA](-)> [HFuA](-), as measured by linear sweep voltammetry. This order can be attributed to the electrons' delocalization in [FuA](-) and in [TpA](-) aromatic anions, which has enhanced the oxidative limit potentials and the overall electrochemical stabilities.