Physicochemical properties of ionic liquids (ILs) can be favorably modified by addition of appropriate cosolvents. Solvatochromic absorbance probes are used to assess physicochemical properties of the mixture of a common IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and propylene carbonate. Parameter E-T(N) obtained from electronic absorbance of Reichardt's betaine dye within the mixture is observed to be higher than that predicted from ideal additive behavior. Absorbance from N,N-diethyl-4-nitroaniline and 4-nitroaniline in concert with absorbance from Reichardt's dye is used to obtain dipolarity/polarizability (pi*), hydrogen-bond donating (HBD) acidity (alpha), and hydrogen-bond accepting (HBA) basicity (beta) of the mixture. While pi* and alpha of the mixture are observed to be higher than those expected from ideal additive behavior, beta is observed to be lower. Solvatochromic absorbance probe behavior suggests solute-specific preferential solvation; FTIR absorbance of the mixture implies negligible specific solvent-solvent interaction(s) within the mixture. Combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) equation is demonstrated to predict the solvatochromic parameters satisfactorily. (C) 2008 Elsevier B.V. All rights reserved.