A series of binary mixtures ranging from 0 vol.% to 100 vol.% CH3CN in BMIM-PF6 were investigated to identify an optimum ratio for use in electrochemical studies involving CO2. Density, viscosity and conductivity were measured for the range of binary mixtures and compared to previously published data. The electrochemistry of a model compound, azobenzene, was studied as well. The data indicated that a binary mixture containing 15-20 vol.% (approximately 0.5 mol fraction) CH3CN in BMIM-PF6 was optimal for electrochemistry, and FTIR of CO2 saturated solutions demonstrated that the solubility of CO2 in the 15-20% CH3CN mixtures was only about 10% lower than that seen in the neat BMIM-PF6. (C) 2011 Elsevier Ltd. All rights reserved.