We have characterized a ternary mixture of N-methyl-(n-butyl)pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 14 TFSI) + 0.5 M LiTFSI + y poly(ethylene glycol) dimethyl ether (PEGDME) (y = kg PEGDME/kg PYR 14 TFSI) as an electrolyte in Li metal/S cells. The presence of PYR 14 TFSI in the mixture resulted in a significant improvement of the thermal stability and the ionic conductivity (σ) of the mixture with increasing PEGDME contents (for example, σ = 4.2 × 10 -3 S cm -1 at 29 • C for y = 2.0). These improvements are most significant at low temperatures, which is probably due to a lowering of the viscosity of the mixture with higher amounts of PEGDME. It is found that the mixture has good compatibility with respect to Li metal as demonstrated by time-dependent interfacial impedance and galvanostatic Li stripping/deposition measurements. We found that a Li/S cell with PYR 14 TFSI + 0.5 M LiTFSI + y PEGDME (y = 2.0) can deliver about 1300 mAh g -1 -sulfur at 0.054 mA cm -2 at ambient temperature at the first cycle. A better charge/discharge cyclability of the Li/S cell in PYR 14 TFSI + 0.5 M LiTFSI + y PEGDME was found at higher PEGDME contents, and a Li/S cell with the mixture having y = 2.0 exhibited a capacity fading rate of 0.42% per cycle over 100 cycles at 0.054 mA cm -2 at 40 • C. Consequently the PYR 14 TFSI + LiTFSI + PEGDME mixture is a promising electrolyte for Li/S cells.