In order to explore the various possible property trends in ionic liquid mixtures, five different ionic liquids were mixed with N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C(3)mpyr][NTf2]), and the viscosities, excess molar volumes, ionic conductivities, and phase diagrams of the mixtures were determined over a range of temperatures. In a number of the mixtures the crystallization of both components was completely suppressed and no melting point was observable. Such mixtures of similar ionic liquids thus have potential for use in low-temperature applications by extending the liquid range to T-g. The molar conductivities and viscosities are described as approximating predictable or "simple" mixing behaviors, while excess molar volumes were found to show a variety of mixing and nonideal mixing effects. Mixture equations for viscosity and conductivity are discussed and analyzed. An immiscibility window was observed in the trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P-6,P-6,P-6,P-14][NTf2]) in the [C(3)mpyr][NTf2] system in the [C(3)mpyr][NTf2]-rich region. Unusual physical properties are exhibited by miscible compositions near the demixing line. These compositions are described as [P-6,P-6,P-6,P-14][NTf2]-like, even up to 0.5 mol fraction of [C(3)mpyr][NTf2].