In situ FTIR spectroscopy has been utilized to investigate the partitioning of a model solute between 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) room temperature ionic liquid (RTIL) and high-pressure CO2. A combined ATR-IR and transmission spectroscopic technique has been developed to calculate the partition coefficient, K-c, of benzil between the two media as a function of CO2 pressure. Analysis of the data acquired [K-c(RTIL/CO2) = 43.8 at a temperature of 40 degrees C and 10.5 MPa] illustrate that in situ spectroscopy is a viable method for obtaining partitioning data for solutes in RTIL/CO2 systems. (c) 2005 Elsevier Ltd. All rights reserved.