The synergistic effect on the physical and electrochemical properties derived from mixing bis(trifluoromethanesulfonyl)imide-based, TFSI−, and (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide-based, IM14 −, ionic liquids (ILs) with a common N-butyl-N-methylpyrrolidinium, PYR14 +, cation has been examined. The incorporation of even small mole fractions (x ≤0.3) of PYR14IM14 into PYR14TFSI is capable of strongly hindering the ability of the mixtures to crystallize. The lowering of the melting point caused by PYR14IM14 addition, in conjunction with the relatively high conductivity of PYR14TFSI, results in an ionic conductivity for all of the mixtures approaching 10−4 and 10−3 Scm−1 at −20°C and 20°C, respectively. The PYR14TFSI/PYR14IM14 binary mixtures may therefore be appealing for electrolyte applications in electrochemical devices operating at low temperatures.