Ammonium- and phosphonium-based polyoxometalate ionic liquids (POM-ILs) have been synthesized and characterized. The results of small-angle XRD indicate that phosphonium-based POM-IL possesses a layered type structure. They are temperature control-type POM-ILs and can exhibit a phase transformation below 100 °C. Phosphonium-based POM-IL exhibits higher thermal stability and conductivity than its ammonium analogue.