Although aqueous biphasic systems (ABS) composed of phosphonium-based ionic liquids (ILs) have demonstrated superior performance as viable media for biocompatible extraction processes, the formation of ABS with tetrabutylphosphonium trifluoroacetate ([P-4444]CF3COO) and tributyloctylphosphonium bromide ([P-4448]Br) were seldom investigated. To evaluate the hydrophilicity of [P-4444]CF3COO and [P-4448]Br, the solubility curves for the mixture of the ILs and water were determined. The influence of temperature on the phase behavior for the [P-4448]Br + K3PO4 + H2O and [P-4444] CF3COO + K3PO4 + H2O systems were also obtained. Results show that both [P-4448]Br + H2O and [P-4444]CF3COO + H2O binary systems undergo lower critical solution temperature-type phase transition, and the ABS composed of [P-4444]CF3COO and [P-4448]Br can be formed with K3PO4 at a wide range of temperatures. Moreover, the ABS are highly temperature dependent, and biphasic region expands with an increase in temperature. On the other hand, the capability of the ILs to induce ABS follows the tread [P-4448]Br > [P-4444]CF3COO. This suggests that the lower cation hydro philicity of ILs results in greater biphasic area in the ABS formed by phosphonium-based ILs.