The phase behavior of imidazolium-based ionic liquid (IL) + phosphonium-based IL binary mixtures with a common anion has been investigated in the temperature range 253-453 K. Bis(trifluoromethanesulfonyl)amide ([NTf2](-)) and tetrafluoroborate ([BF4](-)) were selected as common anions. The imidazolium-based IL + phosphonium-based IL mixtures with a common anion showed an upper critical solution temperature (UCST). The phase separation temperatures of the mixtures decreased with increasing alkyl-chain length of the imidazolium cation. In contrast, the phase separation temperatures of the mixtures increased as the alkyl-chain length of the phosphonium cation increased. The phase separation temperatures of the imidazolium-based IL + phosphonium-based IL mixtures with [BF4]as a common anion were remarkably higher than those with [NTf2](-).